5.7: Phylum Chordata - Biology

Four diagnostic features characterize species in the phylum Chordata: 1) The notochordis a malleable rod running the length of the organism’s body, to which the rest of the skeletal structure relies upon for foundational support; 2) the presence of a tail extending past the anus; 3) a hollow, dorsal nerve cord (becomes the spinal cord in humans!); and 4) pharyngeal gill slits, with the ability to be modified for specialized functions in mature vertebrates. Though this might sound outlandish, but since we are chordates, even all human fetuses early in development have gills!

“9-Week Human Embryo, 2000” by Ed Uthman [CC by 2.0]

There are many classes that comprise the phylum Chordata. These classifications and their relationships are constantly reassessed with taxonomic research and our continued development of our understanding of life on Earth. This chapter will focus primarily on the marine chordates.


The subphylum Urochordata includes the tunicates, otherwise known as ‘sea squirts’. They are exclusively found in marine environments and it seems strange that they are in the same category as vertebrates. Even more interesting is their manner of nutrient collection and waste expulsion. It is a rather simple system, there is an incoming siphon, that draws in water and food particulates that may be floating in the water. These nutrient particulates are then passed down to the intestine where they are processed for sustenance. The excess water and waste products are expelled through the other siphon known as the excurrent siphon. Even more surprising is the fact that these animals only show all traits of Chordates when they are in larval stage. Urochordata often occur in colonial form as adults as seen below.

“Komodo Tunicate, 2006” by Nick Hobgood [CC by 3.0]


The subphylum Cephalochordata (lancelets) exhibit all the traits of Chordata as adults. Like Urochordata, they are also marine organisms and may be found world wide in shallow waters. They are often observed in benthic environments, where they burrow themselves into the sediment but leave their anterior exposed as a foraging mechanism [1]. The anterior portion of these organisms resembles that of the face of shrimp or of a praying mantis. Their manner of feeding is through the filtration of nutrient rich waters around them. Cephalochordata have been described as ‘fishlike’ in comparison to their Urochordata counterparts. The species that comprise this subphylum are relatively small ranging from 5-15 cm in length. And though they possess a closed circulatory system, they have no heart. Instead, their blood is oxygenated via their gill slits and recycled throughout. Their dorsal nerve runs throughout their body, however the anterior end does not form a brain complex.

“Branchiostoma lanceolatum, 1997” by Hans Hillewaert [CC by 4.0]


Moving into the vertebrate category, it is important to start off with the superclass Agnatha, more commonly known as the jawless fish. Two of the classes comprising Agnatha are Cyclostomata (lampreys) and Myxini (hagfishes). It is commonly accepted that the evolution of vertebrates began with the segmentation of a vertebral column. This gave rise to a ‘backbone’ [1]. This vertebral column typically incorporates and/or replaces the primitive notochord. Further vertebrate adaptations include the development of sensory organs, a complex neural system, and a brain encased in a skull. Vertebrate Chordates also exhibit bilateral symmetry, having a closed circulatory system with a chambered heart. The degree to which the heart is chambered (i.e. one, two, three and four chambered hearts), varies with taxonomic class.

Lampreys (Cyclostomata) are jawless fish that are parasitic on other fish. As juveniles they derive their nutrition from filter feeding on plankton and particles floating in fresh water. Lamprey juveniles wait until maturity to migrate into salt water environments. In the ocean, lampreys may often be observed attached to larger reef fish and megafauna. They press their mouth to their host and using a tongue, they draw blood and tissue out of their victims.

“Sea Lamprey, 2015” by Joanna Gilkeson [CC by 2.0]

The Myxini Class are commonly known as hagfish and reside solely in marine environments. They are similar in structure to eels and are jawless. Myxini can be either consumers or detritivores, feeding on the flesh of weakened or already dead fish. They have also been known to prey upon small invertebrates. Just like the lamprey, hagfish tongues resemble a rasp, and are similar to serrations on a knife. One of the defense mechanisms exhibited by the Class Myxini resembles that of many amphibians in that when threatened, they will release mass amounts of high-viscosity fluids. These fluids help to distract, escape from, confuse or deter potential predators [1].

“Eptatretus soutii, 2005” by Stan Shebs [CC by 3.0]

Osteichthyes and Chondrichthyes

Fish can be divided into the two main groups, Osteichthyes and Chondrichthyes. Osteichthyes, traditionally considered as an taxonomic class, is now known to be a paraphyletic group. As their name suggests, they are boney fish that dwell in both fresh water and salt water around the globe. Boney fish are comprised of a hard calcareous skeleton and are coated in slippery, sometimes sharp scales. An important feature of boney fish is their lateral line. This is a zone that runs horizontally along the body of the fish and is predominantly used in the detection of vibrations. The lateral line has been attributed to the coordinated and navigational success of schools of fish, in which mass quantities of individuals conglomerate for various reasons. There are currently over 34,000 known species of fish on the planet, and that number is both growing and shrinking [fishbase]. Many fish populations are threatened by egregious over-fishing practices which has caused species diversity and abundance in various ecosystems to move into a downward spiral.

“1. Oncorhynchus mykiss 2. Pontinus nematophthalmus 3. Aracana aurita 4. Peristedion gracile” by Public Domain [CC 0]

The skeletons of Chondrichthyes are comprised of cartilage rather than bone. Cartilaginous fish include the Rays, Sharks and Chimaeras. The vast majority of sharks are predators, much of their power and deadliness comes from their evolutionary adaptations in their physiology. Their streamlined and highly muscular body lends to their high proficiency as consumers. The flattened bodies of rays contributes to their free-flowing nature through water. Often times rays consume invertebrates that are found in the benthos of the ocean, however they are highly diverse in size, morphology and behavior. Giant Manta Rays can get up to 7m in width and filter feed on masses of zooplankton, whereas the southern stingray may only be a few dozen centimeters in width and feed on the bottom in sandy flats near coral reefs [NOAA].

“Hypanus americanus, 1992” by Barry Peters [CC by 2.0]


The Chimaera are an interesting group of organisms that inhabit the deep sea. Their physiology is atypical and they possess a cross of several characteristics as their name suggests. Rather than a jawline filled with teeth Chimaeras have a flat dental plate. Origins of Chimaeroid marine species can be traced back upwards to 280 million years, predating the earliest dinosaurs of the Triassic period.

“Dunkleosteus Skull, 2014” by Zachi Evenor, courtesy of Vienna History Museum, Austria [CC by 3.0]

A January 2017 discovery of a fossilized Dwykaselachus oosthuizeni skull showed that there are few structural differences in ancient Chimaeras compared to modern Chimaeras. CT scans of the fossil showed significant cranial nerves, inner-ear structure, and nostrils, which are all exhibited by modern Chimaeras [UChicago Medicine]. An incredible aspect of this would pertain to Earth’s projected geological history. The presence of modern-day Chimaeras with little difference from ancient species means that these organisms survived two mass extinction events, showing the perseverance of the beings that dwell in the ocean depths.

“Modern Deep Sea Chimaera” by NOAA, Public Domain [CC 0]

In Text Reference: 1. Peachey, Donna & Gordon, The Biocam Museum of Life Series. Kelowna, B.C. Canada VIY 7N8 Box 417 PBC, 2000

The information in this chapter in thanks to content contributions from Jason Charbonneau

Study Notes on Integument in Mammals | Phylum Chordata

The integument or skin in mammals as well as in all vertebrates is continuous with the mucous mem­brane of mouth, rectum, urinogenital organs, nostrils and eyelids. The skin is made up of two distinct and embryo-logically different layers.

The outer layer is epidermis and it is ectodermal in origin and the inner layer is dermis which is mesodermal in origin. The two layers are sepa­rated by a basement membrane (Fig. 10.4).

The outer layer or the epidermis is again divided into a number of distinct strata. The innermost layer of the epidermis is called stra­tum germinativum or Malpighian layer. This layer is made up of tall and columnar cells arranged perpendicular to the dermis.

The cells divide mitotically and continually. The new cells thus formed tend always to reach the surface and on their sojourn become flattened and show poor stain ability.

The layer formed by these flat cells is known as transitional layer. The outermost layer of the epidermis is called stratum corneum or horny layer. The cells of this layer are flat and dead. The chief constituent of these cells is keratin which is a very hard, tough and insoluble protein.

The epidermis in certain parts of the body of man is a bit different. In the thick skin on the soles of the feet and palms of the hands the transition from Malpighian layer to corneal layer is not so abrupt. The transitional layer in these parts of the body is further subdivided into an inner stratum granulosum and an outer stratum lucidum.

The thickness of the epidermal part of the skin remains fairly constant because the rate of proliferation of the stratum germinativum is nearly equal to the loss of corneal cells. The dermis or corium is thicker than the epidermis and is made up of connective tissue fibres, smooth muscle fibres, blood vessels, nerves and glands.

In whales and seals the fat forms a thick layer, called blubber which acts as food reservoir and also helps in maintain­ing body temperature. The pigments of the skin or melanin in mammals never remain confined in spe­cialised cells but they remain in the cells of the deepest layer of the epidermis.

Functions of the Integument in Vertebrates:

i) The integument or skin protects the body from the entry of foreign bodies and prevent from the mechanical injuries.

ii) The hard dermal and epidermal scales that protect the skin from surface abrasion and also the soft tissues which lie beneath it.

iii) Hair, bristles and spines are employed for offensive and defensive purposes.

iv) The impervious integument helps the body from loss of water.

2. Thermoregulation:

The integument of warm-blooded animals regulates the body temperature. Feathers of birds, sweat glands and blubber of mammals help in the regulation of body temperature. Deep covering of the hairs help in the conser­vation of heat, specially during winter.

3. Storage of food:

In whales, seals and sea cows, a sub- dermal fat layer forms a thick layer, called blubber, which acts as food storage.

The integument of some aquatic verte­brates (e.g., aquatic amphibians) serves as an organ for excretion. During ecdysis the waste material which is stored in the corneal layer of the skin is shed. Sweat of the sweat glands aids in removing nitrogenous wastes from the body.

The moist skin of common eel, mud skip­pers and swamp eels sub-serve respiration. The skin of amphibians is moist and highly glan­dular that help air in contact with the skin to be interchanged and thus performs accessory respiration. In plethodontid salamanders, the lungs are absent, so rely totally on cutaneous respiration.

The skin acts as an organ of secretion. The different glands are located in the skin those help the vertebrates in different ways for sur­vival. Fishes possess numerous mucous glands in the skin that secrete abundant mucous.

The slimy mucus of the fish on the skin reduces resistance during swimming. The poison glands of fishes, amphibians and snakes are used for protection and predation. Mammary glands, scent glands, and sebaceous glands are present in the skin and serve different functions.

Various types of integumentary derivatives sub-serve different types of locomotion’s. The fins of fishes, web in aquatic amphibians, terrapins and aquatic birds, scutes in snakes, adhesive pads in climbing lizards, feathers in birds and patagium in flying lizards help in different modes of locomotion.

Migration of Fishes | Phylum Chordata

Migration of fish is defined as a class of movement which involves a long journey to a definite area for some purpose and impels the migrants to return to the region from which they have migrated. The purpose of the journey is breeding and feeding.

Migration is a two-way journey. It includes emigration (outward journey) and immigration (return journey or inward jour­ney). The fishes are notable for migration for the purpose of spawning. The inherent pur­pose of migration is not known.

2. List of Migratory Fishes:

The following fishes may be mentioned as migratory fishes:

Common name – Scientific name

a. Lampreys – Petromyzon marinus, Entesphenus, Lethenteron, Ceotria, Mondacia

b. Eel – Indian longfineel (Anguillabengalensis bengalensis), Short- fin eel (Anguilla bicolor bicolor), Common freshwater or European eel (Anguilla anguil­la and A. vulgaris), American eel (Anguilla rostrata), etc.

c. Hilsa shad – Hilsa (Tenualosa) ilisha

d. Toli shad – Hilsa (Tenualosa) toli

f. Salmon – Salmo salar, Oncorhynchus

3. Types of Migration:

Myers (1949) has classified the following types of fish migration.

a. Diadromous migration:

When the migrations occur in between freshwater and marine environments.

Diadromous type of migration can be divided into following three-types.

(i) Anadromous migration:

When migration occurs from sea to fresh­water for spawning, called anadromous migration e.g., Atlantic salmon (Salmo salar), Hilsa shad (Tenualosa ilisha), Toli shad (Tenualosa toli), Paradise fish (Polynemus paradiseus), Flat head sillago (Sillaginopsis panijus), Sturgeon (Acipenser) and Salmon (Oncorhynchus).

(ii) Catadromous migration:

The journey of freshwater fishes to the sea for spawning, called catadromous migration e.g., Indian longfin eel (Anguilla bengalensis bengalensis), Shortfin eel (Anguilla bicolor bicolor), Common freshwater or European eel (Anguilla anguilla, A. vulgaris), American eel (Anguilla rostrata).

(iii) Amphidromous migration:

Migration of fishes from freshwater to sea and vice versa and is not for the purpose of breeding but for the other purposes (e.g., food). The amphidromous fishes migrate regu­larly at some particular stage of the life cycle.

Marine amphidromy occurs in flat head mullets (Mugil cephalus) which spawn in the Indian seas during autumn and early winter and whose young stage spend a short period in brackish water and freshwater. They are able to survive in ponds with salinity at 87%. After spending in fresh- or brackish water they return to marine water.

b. Potamodromous migration:

Migrations of fish that occur entirely in freshwater, called potamodromous e.g., carps and trout. Trouts and carps travel long dis­tances in large shoals in search of suitable spawning grounds and return to feeding areas after spawning.

c. Oceanodromous migration:

Migration which occurs entirely in sea, called oceanodromous migration. Horizontal and vertical distribution is considered in oceanodromous migration. Many fishes undertake short distance migrations through­out their life and some fishes like herrings, cod, tuna and plaice, cover long distance migrations.

Again on the basis of the purpose, migra­tion in fish has been classified into following types:

(i) Alimental migration:

Migration occurring for the purpose of food procurement, e.g., Bombay duck (Harpadon).

(ii) Gametic or Spawning migration:

Migration by a fish for the purpose of reproduction which enables the species for better survival and proper development of the eggs, e.g., Tenualosa ilisha.

(iii) Climatic migration:

Migration takes place for the purpose of reaching a particular region to secure better climatic conditions, e.g., salmon and sturgeon etc. Swordfish (Xiphius glodius) living in tropi­cal and temperate waters, sometimes migrate north in spring and autumn to cold waters, for the suitable climatic condition.

4. European Eel Migration:

Catadromous migration (Gk. kata = down, dramein = to run)

The life-history of common river eel or the European eel (Anguilla anguilla or A. vulgaris), and American eel (Anguilla rostrata) will repre­sent a clear idea about the catadromous migra­tion. The common river eels or European eels are found along the shores of Europe and in inland waters of countries near the shores of Europe, some of which inhabit in Iceland, the Mediterranean countries, black sea and the red sea.

The adult eels with their sexual products are not encountered in freshwater, so their biology of reproduction was quite unknown for centuries, From the time of Aristotle the diffe­rent ichthyologists tried to find out the exact spawning ground of the European river eel.

At last Danish ichthyologist Johannes Schmidt who started his investigation in 1904, ultimate­ly succeeded in 1922 to locate the spawning place of the European river eel. It was found in the Sargasso sea of the Atlantic Ocean.

The life history of the European eel is divided into 4 phases (Fig. 6.109):

a. An ordinary yellow eel representing the growing and feeding form in the river.

b. Changes of the yellow eel into the sil­very eel ready for seaward migration for spawning (breeding phase).

c. A pelagic larval phase and

d. The metamorphosis of the pelagic lar­val phase to elver or young eels.

The yellow coloured variety living in fresh water represents the feeding and growing forms. With the advent of autumn, majority of yellow eels become silvery and prepare to undertake migration forwards the spawning ground, the Sargasso sea of Atlantic Ocean. The eels in the river spend about 10 to 12 years feeding partially on fish.

During the transformation from yellow- coloured to silvery colour, the yellow eels stop feeding, eyes become greatly enlarged, the snout becomes sharper with thinner lips and the yellow colouration is replaced by a metallic silvery colour. The silver eels are recognised by having matured organs and shrunken digestive tract.

The primary development of gonads is the stimulus for the beginning of migration. The size of the eggs in the ovaries changes before migration. These silver eels first migrate down to the mouth of the rivers and then into the Atlantic Ocean. The European eels probably migrate over 6000 km between its freshwater feeding streams and its spawning ground.

The spawning ground is located in the western part of tropical waters of Atlantic Ocean between 22° and 33° N. lati­tude and 48° and 65° W. longitude, near Bermuda Islands. Spawning takes place from the end of winter to the middle of summer. After the completion of spawning the parents die.

Eggs are laid in spring at depth of 500 to 700 meters with temperature ranging between 10-12°C. The fertilized eggs float for some time and the youngs hatch out as the pelagic larvae. The larvae are called Leptocephali.

The leptocephali are flat, glassy, leaf-like body. These tiny creatures are provided with elon­gated needle-like teeth for feeding. The gut has a straight tubular structure. The eyes are large and silvery. They now begin their long home­ward journey. At the end of first summer when they become about 25 mm in length on aver­age, are recorded in the Western Atlantic.

By the second summer the leptocephali reach central Atlantic and the size is about 50-52 mm in length on average. In the third summer they become about 72-75 mm in length and reach in the continental shelf of Europe. The larvae of eel are passively drifted by warm water current of the Atlantic Ocean.

During autumn and winter of the 3rd year the leptocephali metamorphose to form elvers or glass eels or young eels. During metamor­phosis the larvae stop feeding, their flattened body become cylindrical and the needle-like teeth is replaced by new ones. The young eels when become three years old, measure about 15-20 cm long.

Then they congregate in the mouths of rivers. Here the elvers or young eels ascend the rivers, grow in size and change their colour into yellow. The males like to stay in the estuaries and the females ascend the rivers in shoals, specially at night during the spring to reach a suitable resting place. In the yellow eels teeth are lost, intes­tine shortens and the anal aperture moves for­ward.

The yellow eels spend 8-10 years on fee­ding and growing and on maturity, change into silver eels and start their perilous journey towards the Sargasso sea. The well fed silver eels first stop their feeding and leaving the rivers, empty into the Baltic Sea and gradually in the abyssal depths of the Atlantic Ocean.

Among Indian fish, hilsa or Indian shad (Tenualosa ilisha) represents an example of anadromous migration. Its occurrence is recorded in the coastal waters of Pakistan, India, Sri Lanka, Bangladesh and Myanmar. Its presence in the estuaries, rivers and lakes (e.g., Chilka), mostly during spawning season indi­cates its anadromous migration.

During spaw­ning season the hilsa population ascends the rivers of the Hooghly, the Ganges, the Mahanadi, the Godavari, the Krishna and the Kaveri and its tributaries in the eastern region of India, and the Narmada, the Tapti and the Kali in the Saurastra coast of the west India.

Around the Indian coasts, the Hooghly-Matla estuarine system covers a major portion of the Ganga-Brahmaputra delta and is estimated to be 3,100 sq. miles which is the largest in India.

This estuary zone is mostly famous for hilsa migration. Before the con­struction of Farakka barrage on the Ganges in West Bengal the hilsa used to migrate up to Delhi during the spawning season covering a distance over 1,250 km. In Bangladesh it ascends in the rivers of Meghna, Padma, Brahmaputra and its deltaic rivers. It also migrates in the upper part of Irrawaddy river of Myanmar.

The marine distribution of hilsa in the Bay of Bengal and in the Arabian Sea is due to seve­ral factors, such as vast stretch of continental shelf, low salinity, discharge of huge amount of monsoon freshwater in the coastal region through the rivers, monsoon winds, huge abundance of planktons as food, and other favourable hydrological parameters. Oceanic properties along the Indian coasts are given below in Table 24.

In the sea, hilsa population is found along the east coast in the vicinity of rivers before the spawning season but in Gujarat and Maharashtra the hilsa is found about 12-16 km off the coast at a depth of about 20 fathoms.

There are two types of hilsa stocks — one estuarine and offshore stocks which are found in the lower region of the estuary and the fore­shore areas of the seas, and another is riverine stock which spends throughout the year in the river, mainly in the Ganges and the Brahmaputra.

The estuarine stock migrates upwardly during breeding season and after spawning they return to their natural habitat and spend throughout the year till the next breeding season to come. The riverine stock ascends in the more freshwa­ter river zone during breeding season and after spawning they come back to the lower reaches of the river and spend the rest period until the next breeding season comes.

The life cycle of hilsa is divided into 4 phases:

(iii) Fingerling or Juvenile stage and

The size of adults differs in sexes. The females are larger than males. The size of the sexes differs in different seasons, even in the same river. In the Ganges and in the Padma different sizes of sexes have been reported by different authors. In the Hooghly estuary the mature females are recorded about 250 mm in length.

The adult hilsa are laterally com­pressed, fusiform animals, having abdomen with a keel about 30-33 scutes. There are very fine numerous, closely set gill-rakes that indi­cate for planktonic feeding habit, predomi­nantly zooplankton feeder. They mainly con­sume Cyclops, Daphnia, Moina, rotifers and protozoans. The adult hilsa in the freshwater zone of the Hooghly river are column and bottom feeders.

The hilsa is probably polygamous and fer­tilization is external. The riverine stocks of hilsa become mature in between 1-3 years of age. Matured hilsa spawns once in a year but the time of spawning differs in different parts of India. In Hooghly estuary or freshwater region of the Hooghly river spawning takes place at the onset of evening but in the Narmada spawning takes place in the early morning.

During upstream migration schooling beha­viour among hilsa is seen and males move in the upper surface layer of water and females move in the deeper water layer in river during monsoon period.

Many authors reported that during spawning period the hilsa do not feed or stop feeding but Pillay (1958) has reported that the appreciable amount of food is available during spawning period, especially of hilsa of the Hooghly region. The feeding intensity is increased considerably after spawning period.

In the spawning ground both the sexes discharge their garnets in freshwater. The tailed spermatozoa survive few hours in the water and ova are large, translucent at the marginal zone. The fertilized ova turn trans­parent after half an hour. The fertilized eggs float in water and colour is about light gree­nish yellow. The hatched ova transform into larva or fry when they become 20-40 mm in length.

The abdomen of the larva possesses 5-7 pre-ventral scutes. At about 40-45 mm in length, the dark blotches on the lateral side are seen that indicate the fingerling or young hilsa stage. The larvae are surface feeders, predo­minantly zooplanktons which constitute about 70% in their food composition. When the fry attains 100 mm in size they are called finger- lings or young hilsa.

The body of young hilsa becomes laterally compressed with keeled abdomen. The abdomen possesses 30-32 scutes. The body colour is silvery along the sides. A row of dark blotches is seen on the lateral sides of the body.

When the fingerlings become 150 mm in size, called advanced fingerlings or youngs. At that stage they con­sume small shrimps and phytoplanktons. The early fingerlings (about 80 mm) are found in the lower reaches of the river and estuary zones. The youngs or advanced fingerlings (above 150 mm) occur along the foreshore areas of the seas.

Two types of anadromous migrations are seen among Indian hilsa species. Breeding or spawning migration is seen during the south­west monsoon when the Indian and Bangladesh rivers are flooded by the monsoon rain. Another type is winter or spring migra­tion which is influenced by the certain changes of the water temperature and rain, mainly seen in the Gangetic delta.

The correlation between temperature and movement into freshwater may be a reflection of the energetic cost of migration. In the Indus river of Pakistan and Irrawaddy river of Myanmar, the hilsa migration takes place by the molten snow that creates flood in these rivers.

The adult hilsa spends most of the year in their original places except spawning season. The young and adult hilsa can tolerate certain variation of salinity. The Hooghly-Matla estuary is classified as a mixohaline range, in which salinity varies in different zones.

Mystus gulio migrates to the estuarine and freshwater zones of the Hooghly river for spawning, and Pama pama also migrates to the estuarine zone of the Hooghly river for both genetic and tropic reasons. Toli shad (Tenualosa toli) is found in the western coasts of India, ascends in rivers for spawning.

6. Causes for Fish Migration:

Fish migration is related to several factors such as physical, chemical or biological.

The physical factors include temperature of water, rainfall, quality of water, water depths, pressure, light intensity, photoperiod, turbidity, tides and currents.

The chemical factors include pH of water, salinity, dissolve of O2 and CO2, types of dissolved organic and inorganic substances, and taste of water.

iii. Biological factors:

The biological factors are food, attainment of sexual maturity, endocrine behaviour and competitors and predators.

Phylum Chordata

The Chordata is a very large and diverse phylum which has been studied extensively, mainly because it includes the vertebrates. Invertebrate chordates exist, however, and these are important in evolutionary terms. These are sometimes called protochordates, a term which formerly included the Hemichordata too. There are about 45 000 species of chordates, occupying marine, fresh and brackish water and terrestrial habitats. Several groups have evolved flight.

Body plan

Chordates are bilaterally symmetrical, triploblastic, segmented coelomates which demonstrate a deuterostomic pattern of early embryonic development. At some stage in their development they possess: a dorsal tubular nerve cord, formed from an infolding of a strip of dorsal ectoderm a notochord pharyngeal gill-slits and, usually, a post-anal tail. Segmented muscles are found in the body.


Invertebrate chordates are suspension-feeders: food particles are trapped in mucus on elaborately expanded pharyngeal gill-slit systems and wafted into the gut by cilia modern jawless vertebrates are semi-parasites on jawed fishes most jawed vertebrates are herbivorous or carnivorous, macrophagous feeders. Teeth inserted into vertebrate jaws are almost universal. The gut is complete with a mouth and an anus. An endostyle along the floor of the pharynx in invertebrate chordates is the homolog of the vertebrate thyroid gland.


Most chordates are active swimmers, using muscles which act against the antitelescopic notochord or vertebral column. (Many urochordate larvae metamorphose to form sessile adults.) The evolution of median and paired fins in fishes facilitates control of pitching, yawing and rolling. In land vertebrates, the paired fins have evolved to form jointed limbs which act as levers against the substratum: these may be secondarily lost (as in snakes). A number of chordate groups have evolved flight and others have resumed an aquatic habitat, usually using modified paired limbs.


Chordates are characterized by an endoskeleton in the vertebrates the endoskeleton is made of cartilage, normally replaced by bone (consisting primarily of hydrated calcium phosphate and protein). The notochord forms as an antitelescopic, cartilaginous rod dorsal to the gut and ventral to the nerve cord. It contains a gelatinous matrix surrounded by tough connective tissue. In vertebrates, it is partly or completely replaced by vertebrae which develop intersegmentally and surround the nerve cord.

Respiration and vascular system

The pharyngeal gill-slits, used for suspension-feeding in invertebrate chordates, are used for respiration in the more advanced aquatic forms but are present only transiently in the embryo in land vertebrates. Gills develop in fishes and are present in amphibian larvae. Land vertebrates respire using lungs which develop as pouches from the gut (and are the homolog of the fish swimbladder). Most chordates have a high-pressure, closed circulation with a ventral heart. A dorsal vessel conveys blood backwards while a ventral vessel conveys it forwards.

Osmoregulation and excretion

The characteristic excretory organs of vertebrates are the segmental kidneys, replaced in reptiles, birds and mammals by nonsegmental kidneys invertebrate chordates rely on simple diffusion of waste substances (urochordates) or groups of solenocytes (cephalochordates).


The dorsal hollow nerve cord, present at some stage during the chordate’s life history, is characteristic of this phylum. In metamorphosed urochordates the nerve cord is lost. In vertebrates the anterior end of the nerve cord expands to form a brain.


Reproduction is normally sexual, although parthenogenesis is seen in a few species, and some urochordates reproduce asexually by budding to form colonies.


There are three chordate sub-phyla:

The first two are sometimes called the acraniate or invertebrate chordates. The craniates are more popularly termed the vertebrates.

Sub-phylum Urochordata (Tunicata)

The 1300 species of urochordates or tunicates have larvae with the typical chordate structure (including a notochord, a dorsal nerve cord and a segmentally muscled tail). However, after metamorphosis this structure, apart from the gill slits, is lost. The tail with its musculature and the notochord disappear, while the nerve cord is reduced to a round ganglion from which peripheral nerves radiate. The pharynx enlarges to form a branchial basket with numerous ciliated gill-bars covered in mucus. Water is drawn through the mouth, passing through the gill-slits to an atrium. Food particles are filtered out in the mucus wafted by cilia along the gill-bars. They pass to a dorsal epipharyngeal groove and thence back to the gut.

Ventrally in the pharynx is an endostyle, which concentrates iodine and is the homolog of the vertebrate thyroid gland. The body is enclosed in a cellulose ‘test’ or tunic. Urochordates have a heart (which can reverse its direction of beat) and blood vessels. Functional lymphocytes are seen, and rudiments of the vertebrate adaptive immune system have been described. Blood cells include vanadocytes (containing hemovanadium, although this vanadium-containing pigment has not been demonstrated to have a respiratory function). Urochordates are usually hermaphrodite with an ovotestis. Eggs develop into a ‘tadpole’ larva which metamorphoses into an adult. Many multiply asexually by budding to form a colony with a common exhalant opening from the atrium.

There are three classes: the Ascidacea, the Larvacea and the Thaliacea.

Class Ascidiacea: sea-squirts

Usually sessile and bottom-living. They may be solitary or colonial. Examples include Ciona intestinalis and Botryllus sp.

Class Larvacea

Forms with a neotenic retention of some larval features, such as a tail, into adulthood. Oikopleura has an elaborate, secreted ‘house’ used for filter-feeding. An example is Oikopleura dioica.

Class Thaliacea

Salps, solitary or colonial. The test has muscle bands and the atrial opening points posteriorly allowing the salp to swim by jet-propulsion. An example is Salpa sp.

Sub-phylum Cephalochordata

The Cephalochordata comprise two genera. In Branchiostoma lanceolatum (= Amphioxus lanceolatum), the lancelet or amphioxus , the notochord persists throughout life: it extends to the extreme anterior end, beyond the hollow nerve cord which lies dorsal to it the nerve cord does not expand to form a brain and there is little cephalization. The animal lies half-buried in the sand, suspension-feeding particles from the sea water. Cephalochordates are small (<5 cm). A specialized feature of the sub-phylum is the asymmetrical body architecture whereby segmented muscles and nerves alternate on each side of the body. (The second cephalochordate genus is called Asymmetron.)

The pharynx is long with many ciliated gill-bars: the cilia generate a current so that water is drawn into the mouth, passes through the gill-slits where food is filtered out, and then enters the atrium and thence out through the atriopore. Mucus is produced by a ventral endostyle (the homolog of the vertebrate thyroid gland) in the pharynx. Similarities with the feeding patterns of urochordates are seen. Gonads are arranged segmentally: the sexes are separate. Excretion is facilitated by groups of flame cells (solenocytes) similar to those in platyhelminths and polychaete annelids. The blood vessel flow pattern is similar to that in vertebrates, but there are no blood cells nor a heart: the vessel walls are contractile. An example is Branchiostoma lanceolatum (amphioxus or lancelet).

Sub-phylum Craniata (Vertebrata)

Craniates possess the diagnostic chordate features (at some stage in their life histories) of a notochord, hollow dorsal nerve cord, pharyngeal gill-slits and a post-anal tail. All craniates have an anterior expansion of the dorsal nerve cord, the brain, contained in a cranium (the brain-box or skull). This cephalization is further marked by extensive development of sense organs in the head. The notochord is usually replaced in part or totally by intersegmental cartilage or bony units, the vertebrae. Cartilage is found in some invertebrates such as Mollusca, but bone is unique to vertebrates, mainly consisting of hydrated calcium phosphate minerals in a protein matrix. There is a ventral heart behind the head. Blood flows forward ventrally from the heart, then through the gills, and then upwards and backwards dorsally. The excretory organs, the kidneys, lie dorsally in the coelom: they comprise thousands of nephron units. The sub-phylum is normally divided into two superclasses: the agnathans and the gnathostomes.

Superclass Agnatha (jawless vertebrates)

The earliest known vertebrates, the ostracoderms, lacked jaws: these armored fishes are totally extinct. Modern Agnatha belong to the class Cyclostomata (round mouths): this comprises the lampreys and the hagfishes . The absence of jaws is diagnostic. Cyclostomes have round, suctorial mouths. They are semi-parasites on larger, jawed fishes to which they attach, rasping at the host flesh with a toothed tongue and horny teeth within the mouth cavity. The sexes are separate. The fertilized eggs of lampreys hatch to form filter-feeding ammocoetes larvae which lie in the mud after some years they metamorphose into lampreys.

Specialized features of lampreys include the mouth and tongue, the streamlined shape, and pouched gills allowing a tidal flow of water over them.

Primitive features include a persistent notochord, a lack of paired fins, no ducts from the gonads and seven pairs of gills. The skeleton is cartilaginous and includes a cranium, supports anterior to each gill pouch (gill arches) and peg-like vertebrae.

Degenerate features include reduced eyes and a simple, straight gut. Examples include Lampetra fluviatilis (common lamprey) and Myxine glutinosa (Atlantic hag).

Superclass Gnathostomata (jawed vertebrates)

The Gnathostomata includes all other vertebrates. In gnathostomes the first gillbar becomes ‘wrapped round’ the mouth to form the upper and lower jaws. This allows feeding on large particles (macrophagy), especially when teeth are inserted into the jaws. Eight classes of gnathostome are usually recognized.

(1) Class Acanthodii. The classification of the extinct acanthodians, known from the Devonian, is problematical: they exhibit affinities to cartilaginous fishes, to bony fishes and to forms unknown today. They are the first fishes to have true paired fins (unlike the paddles of some ostracoder magnathans) and to possess jaws. The paired fins were attached to the trunk via limb girdles, and there were intermediate, paired spines, between the pectoral and pelvic fins, apparently not associated with fin structures. An example is Climatius sp.

(2) Class Placodermi. The placoderms were Palaeozoic shark-like fishes with jaws and an armor of bony plates. There were several subclasses, now all extinct. An example is Dunkleosteus sp.

(3) Class Chondrichthyes. First known from the upper Devonian, the Chondrichthyes (cartilaginous fishes) have two subclasses. With the exception of only four or five species they are marine. Chondrichthyes have a cartilaginous skeleton (which may be secondarily calcified) and scales, identical in structure to teeth (including dentine). Pectoral and pelvic paired fins are present. Fertilization is internal, with the females laying large, yolky eggs or exhibiting (ovo)viviparity.

(a) Subclass Elasmobranchii.

The elasmobranchs include the sharks, dogfishes, skates and rays. The upper jaws move with respect to the cranium there are 5–7 gill-slit pairs and an anterior spiracle, each opening separately to the exterior. The order Selachii includes the sharks, many of which are large predators the order Batoidea includes skates and rays with dorso-ventral flattening and enlarged pectoral fins. Many batoideans are molluscivores with flattened, crushing teeth. Examples include Carcharodon carcharias (great white shark) Manta sp. (manta ray).

(b) Subclass Holocephali.

The deep-sea rat-fishes, rabbit-fishes and chimeras may not be closely related to the sharks and rays. Their skeletons, gill and vascular systems, and their nervous systems are different from those of the Elasmobranchii. An example is Hydrolagua colliei (rat-fish).

(4) Class Osteichthyes. With more than 30 000 species, the Osteichthyes (bony

fishes) include more species than all the other vertebrate classes put together. The embryonic cartilage in the skeleton is replaced by bone. Paired fins are attached to limb girdles. The gill-slits open to an opercular cavity which is enclosed within an operculum. Pharyngeal pouches from the foregut were probably primitive lungs: these may be modified to form a swim-bladder, used for buoyancy. Fertilization of the eggs is external: large numbers of small eggs develop into larvae (fry) which metamorphose into adults.

(a) Subclass Actinopterygii.

The Actinopterygii or ray-finned fishes have bones and muscles restricted to the bases of the fins which are supported distally by fin rays. The bony scales have a thick layer of enamel. Almost all bony fishes alive today are actinopterygians: three grades or infraclasses are recognized, with progressively more advanced features:

  • All early actinopterygian fossils belong to a group of Chondrostei known as the Palaeonisciformes (palaeoniscids) which radiated extensively between Carboniferous and Triassic times the modern Chondrostei are relics and include the bichir, the paddle-fishes and sturgeons. Air-sacs or lungs are present. Examples are Polypterus ornatipinnis (bichir) and Huso huso (Volga sturgeon).
  • At this grade, morphological body changes allow for more efficient swimming and feeding. Most Holostei died out in the Cretaceous and the Eocene, but survivors include the bowfin and the gar-pike. Examples are Amia calva (bowfin) Lepisosteus osseus (gar).
  • Most living fishes are teleosts (about 30 000 species): the grade or infraclass radiated extensively during the early Tertiary. Teleosts have thin, bony scales without enamel, a moveable upper jaw which hinges anteriorly to the skull, a complete vertebral column which replaces the notochord and a single, dorsal swimbladder which may or may not be connected to the gut. Teleost classification is complex: many zoologists feel that they show polyphyletic (multiple evolutionary) origins from the Holostei. Examples are Cyprinus carpio (carp) Esox lucius (pike).

(b) Subclass Sarcopterygii.

The sarcopterygians or fleshy-finned fishes have paired fins with an axis of bone and muscle. There are two intra classes:

  • The main importance of the crossopterygians (the tassel-finned fishes) lies in the fact that from them probably arose the land vertebrates (the Tetrapoda). Forms such as Osteolepis sp. have similar limb bone and skull bone arrangements to the earliest amphibians. There is only one living species, the coelacanth. This ovoviviparous fish (from deep waters off the Comoro Islands in the western Indian Ocean and elsewhere) has a primitive blood system and a fat-filled swim-bladder. The living example is Latimeria chalumnae (coelacanth).
  • The true lungfishes survive today in three living species, from Australia, South America and East Africa respectively, being relics of a group abundant in Devonian fresh waters. Dipnoans have lungs and breathe air but they also retain their gills. The African and South American forms are obligate air-breathers, but the Australian lungfish needs oxygenated water to survive. The blood system is, for fishes, sophisticated, with a partially divided heart and with pulmonary veins returning oxygenated blood to the left atrium. Estivation in cocoons the mud is common during drought periods. Male African and South American lungfishes brood eggs which hatch into larvae with external gills. Examples include Neoceratodus forsteri (Australian lungfish) and Lepidosiren paradoxa (South American lungfish).

(5) Class Amphibia. Amphibians arose from the crossopterygian fishes during the early Devonian: these early amphibians were known as Labyrinthodontia on account of the architecture of their teeth. Modern forms, collectively known as Lissamphibia, have problematical relationships with the labyrinthodonts.

Amphibians typically possess two pairs of legs (not fins), although one or both pairs may be secondarily absent. The pectoral girdle does not connect to the back of the skull there is a pelvic girdle connected to the vertebral column by sacral vertebrae. The middle ear has a single bone connecting the eardrum to the inner ear.

Amphibian larvae (tadpoles) have external gills which may later be internalized. They develop in water, which may include specialized habitats such as pools in the bases of flowers or a brood pouch in the mother’s mouth. Metamorphosis is under the control of the hypothalamus– pituitary–thyroid axis and a prolactin–thyroxin balance. Metamorphosis may be profound: lungs replace gills (although many species also employ cutaneous respiration). Some urodele species become sexually mature without undergoing metamorphosis, exhibiting neoteny (retention of larval characters in the adult) and pedomorphosis (sexual maturity in the larva). Amphibian hearts have two atria and one ventricle.

There are three living orders of Lissamphibia:

(a) Order Anura.

The anurans are the toads and frogs which in the adult form lack tails. Sexual maturity is always preceded by metamorphosis from a tadpole larva. Examples are Rana temporaria (common frog) Rana catesbeiana (bullfrog) Xenopus laevis (African clawed toad).

(b) Order Urodela.

The newts and salamanders and their allies have long tails used for swimming. Forms range from those that are totally terrestrial, through facultative neotenes which are sexually mature as larvae and only metamorphose in response to drought or thyroxine treatment, through to perennibranchiates which are never observed to metamorphose. These forms retain their gills. Examples are Triturus cristatus (crested newt) Ambystoma mexicanum (Mexican axolotl).

(c) Order Apoda.

The apodans or gymnophionids are limbless, burrowing amphibians from the tropics or southern temperate regions the larval stage may pass within the egg. An example is Dermophis mexicanus.

(6) Class Reptilia.

Reptiles evolved from labyrinthodont amphibians during the late Devonian. The ancestral group were the Cotylosauria or ‘stemreptiles’. From this group evolved various taxa including the Chelonia (tortoises and turtles), the Lepidosauria (the tuatara, lizards and snakes), the Synapsida (whose later members included the mammal-like reptiles which gave rise to the mammals), the Archosauria (whose living members include the crocodiles but which once claimed the flying pterosaurs and the Ornithischia and the Saurischia, the two ‘dinosaur’ orders) and several extinct groups of marine reptiles (such as ichthyosaurs and plesiosaurs). Like birds and mammals, reptiles have an amniote egg : the embryo develops an amnion, chorion and allantois in addition to a yolk-sac.

The three classes are sometimes grouped as a superclass Amniota. There are no larval stages, and some reptiles (e.g. sea-serpents and adders) are viviparous. The chorioallantois acts as a respiratory organ in the developing, waterproof egg. Gills are absent. Oviparous marine and freshwater species (e.g. turtles and crocodiles) must lay eggs on land. In reptiles the articular bone of the lower jaw articulates with the quadrate bone of the skull . Reptiles have a more-or-less waterproof skin, usually with scales. The kidney develops non segmentally and is known as metanephric (as opposed to the segmental, meso- or opisthonephric kidneys of fishes and amphibians). Reptiles reached their peak during the Mesozoic period. Modern forms belong to four orders, distinguished largely by the architecture of the skull.

(a) Order Rhynchocephalia.

This is a primitive group represented today solely by the rare ‘living fossil’ Sphenodon punctatus, the tuatara from New Zealand. It has primitive skeletal features and may resemble forms ancestral to the lizards.

(b) Order Squamata.

The familiar lizards and snakes have scales (and often plates too). The skull architecture shows characteristic fenestrations for each suborder. Members of the suborder Sauria (lizards) usually have four legs although some lizards (e.g. the slow-worm Anguis fragilis), the ‘wormlizards’ (suborder Amphisbaenia) and the snakes (suborder Serpentes) have lost their limbs and limb girdles (vestigial limbs are occasionally seen in pythons). The skull may be specialized to allow the swallowing of large prey, and some species are venomous. Snakes lack eardrums. Examples include Lacerta vivipara (common lizard) Vipera beris (adder).

(c) Order Chelonia.

In turtles and tortoises, the body is enclosed in a shell of bony plates fused to the ribs and the vertebrae. Limb girdles lie within the shell. A horny beak replaces teeth. The organ systems of chelonians are relatively primitive. An example is Chelonia mydas (green sea turtle).

(d) Order Crocodilia.

Crocodiles, alligators and caimans are the only living survivors of the Archosauria (ruling reptiles) which once included the ‘dinosaurs’, pterosaurs and other extinct reptiles of the Mesozoic period. The skin of crocodilians is covered with bony plates with horny scales outside. The lungs are more complex than the simple sacs seen in other reptiles, and the ventricle of the heart is almost completely divided to make an effective four-chambered heart. The skull architecture resembles that of the ‘dinosaur’ groups and birds. An example is Alligator mississippiensis (Mississippi alligator).

(7) Class Aves. In evolutionary terms, the birds are a very homogeneous class, probably derived from saurischian archosaurs (lizard-hipped ‘dinosaurs’) of the Jurassic. The skeletons of the earliest birds (e.g. Archaeopteryx lithographica) show remarkable similarities to coelurosaur ‘dinosaurs’ such as Deinonychus sp. The reptilian scales have evolved into feathers in birds. The forelimb is modified to form a wing, bearing primary feathers on the ‘hand’ for propulsion, secondary feathers on the forearm for lift and contour feathers for aerodynamic streamlining.

Down feathers provide insulation to maintain the homoiothermy (‘warm-bloodedness’) of this class. The skull has a large cranial vault to contain the big brain. The eye orbits are large (birds use vision as their main sense). There are no teeth in modern birds but there is a horny beak whose form reflects the diet of the species. The lungs are large and efficient an associated air-sac system permits a unidirectional air-flow in the lungs, with blood vessels arranged in a crosscurrent fashion to permit extraction of up to 90% of the oxygen from inspired air. The heart has four chambers: there are separate, parallel systemic and pulmonary circulations. All birds are oviparous: eggs are laid on land, usually in a nest. Parental care of the young is normal.

There are two modern superorders:

(a) Superorder Palaeognathae.

The ratites are usually flightless. The palate is specialized. There is considerable debate over whether ratites are ancestral and/or more primitive than other living birds. Examples include Struthio camelus (ostrich) Apteryx haastii (kiwi).

(b) Superorder Neognathae.

This superorder includes all other birds. There are about 20 orders, the largest being the Passeriformes (perching birds, e.g. the robin). Examples include Gallus domesticus [domestic fowl (chicken)] and Corvus corone (carrion crow).

(8) Class Mammalia

The mammals are characterized by the possession of hair (fur): scales (e.g. on a rat’s tail) are rare. The air trapped within the fur provides insulation to help to maintain the homoiothermy of this class. The only bone in the lower jaw is the dentary: this articulates with the squamosal bone of the skull the redundant articulating bones of the reptile are used as auditory ossicles of the middle ear (the maleus and the incus, together with the stapes, hammer, anvil and stirrup).

Unless secondarily lost, teeth are present in the upper and the lower jaws: these show considerable diversity of form, adapted to diet. The young of mammals are suckled on milk secreted by the mother’s mammary glands. The lungs have alveoli. A tidal flow of air into and out of the lungs is assisted by a muscular diaphragm which separates the thoracic and abdominal cavities. The heart has four chambers: as in birds there are separate pulmonary and systemic circulations. Mammals evolved in the Triassic from synapsid reptiles. There is a well documented fossil series which leads from mammal-like reptiles (e.g. cynodonts and ictidosaurs) through to the true mammals. Fossil Mesozoic mammals are rare and are usually known from only jaws and teeth.

(a) Subclass Prototheria (= Monotremata).

This subclass, with uncertain affinity to both fossil forms and to other modern mammals, lays eggs and the milk is produced by glands rather different from other mammals’ mammary glands. The skeleton is distinctive. There are three living species, the duck-billed platypus and the spiny anteaters, all from Australasia. An example is Tachyglossus aculeatus (short-nosed echidna).

(b) Subclass Theria.

The Theria (‘beasts’) includes all other living mammals. There are two infraclasses:

  • Infraclass Metatheria (= Marsupialia). In marsupials, the young are born very immature they crawl through the mother’s fur to the mother’s abdomen where they attach to a nipple, usually in a pouch. Most marsupials are native to Australia where the infraclass has radiated widely, but the opossums are indigenous to the Americas. Marsupials have epipubic bones on the pelvic girdle. An example is Setonix brachyurus (quokka).
  • Infraclass Eutheria (= Placentalia). In placentals, the young grow within the uterus, using a chorioallantoic placenta. The degree of maturity at birth varies (cf. newborn mice and guinea-pigs). The infraclass has radiated widely. About 15 living orders are recognized, including:
  1. Insectivora

A primitive, arguably ancestral order retaining a vestigial cloaca and having a rather lower body temperature than other placentals. An example is Sorex araneus (common shrew).

Forelimbs are extensively modified to form a wing of skin. An example is Pipistrellus pipistrellus (common pipistrelle).

Large eyes and brains, and with prehensile hands and (often) feet. Opposable thumb and the claws replaced by nails. Otherwise rather unspecialized: includes prosimians (e.g. lemurs and bushbabies), old and new world monkeys, apes and humans. Examples include Nycticebus coucang (slow loris) Macaca mulatta (rhesus monkey) Pan troglodytes (chimpanzee).

Normally flesh-eaters, usually with four clawed digits on each limb. Examples include Panthera leo (lion) Odobenus rosmarus (walrus).

Whales and dolphins are wholly aquatic the hind limbs are lost. Some have teeth, others filter plankton with baleen plates on the palate. An example is Tursiops truncatus (bottle-nosed dophin).

The elephants have reduced numbers of teeth the upper incisors are modified to form tusks, and the nose and upper lip are extended to form a trunk. An example is Loxodonta africana (African elephant). [Arguably related orders include the Hyracoidea (the hyraxes) and the Sirenia (the dugongs and manatees: marine herbivores.]

The even-toed ungulates, usually with two hoofed digits on each limb. They are omnivores or herbivores, including ruminants with compound stomachs adapted to the symbiotic digestion of cellulose. Horns are common. Examples include Cervus elaphus (red deer), and Camelus dromedarius (Arabian camel).

The odd-toed ungulates are herbivores with an odd number of hoofed digits on each limb, e.g. one in horses. Examples include Equus berchelli (zebra) Diceros bicornis (African black rhinoceros).

The rodents are a very large and successful order (>3000 species). They have a single pair of upper and lower incisors for gnawing: the teeth grow continuously. There is a wide gap (diastema) between the incisors and grinding molars. An example is Rattus rattus (black rat).

The rabbits, hares and pikas are similar to the rodents but possess two pairs of upper incisors. An example is Oryctolagus cuniculus (European rabbit). There are also three orders of edentates (armadilloes, sloths, anteaters, pangolins and the aardvark) which tend to eat ants, termites and other insects: they are characterized by long snouts, long, adhesive tongues and reduced or absent teeth.

Hemichordata & Chordata classification & examples

Hemichordates are widely distributed in shallow, marine and tropical waters. They are present in deep and cold waters. They have soft and wormlike bodies. They have epidermal nervous system. Most hemichordates have pharyngeal slits. They have three classes.

Class Enteropneusta

  1. They live in shallow water.
  2. They are worm like animals.
  3. They live in burrows on sandy shorelines.
  4. Their body is divided into three regions: proboscis, collar and trunk.
  5. They have about 70 species.

Example: Acorn worms (Balanoglossus, Saccoglossus)

Class Pterobranchia

  1. They are with or without pharyngeal slits.
  2. They have two or more arms.
  3. They are often colonial.
  4. They live in an externally secreted encasement.
  5. They have about 20 species

Example: Rhabdopleura

Class Planctosphaeroidea

  1. They have spherical body. The body surface is covered by ciliary bands.
  2. They have U-shaped digestive tract.
  3. Coelom is poorly developed.
  4. They are planktonic.
  5. Only one species exist

Example: Planctosphaertara pelagica

Phylum Chordata

  1. Notochord is present in them.
  2. They have pharyngeal slits.
  3. Dorsal tubular nerve cord is present in them.
  4. Postanal tail is developed at some time in the chordates life.

Thave bout 45,000 species. Phylum chordata have three sub phylums.

A. Subphylum Urochordata

Notochord, nerve and postanal tail is present only in free swimming larvae. The adults are sessile or planktonic. Their body is enclosed in a tunic that contains some cellulose. Mostly they are marine. Example: Sea squirts or tunicates.

1. Class Ascidiacea: They all are sessile as adults. They may be solitary or colonial. The colony members are interconnected by stolons.

2. Class Appendicularia (Larvacea): They are planktonic. The adults retain tail and notochord. They lack a cellulose tunic. The epithelium secretes a gelatinous covering of the body.

3. Class Sorberacea: They are ascidian-like urochordates. They possess dorsal nerve cords. They live in deep water. They are carnivorous. Example: Octacnemus

4. Class Thaliacae: They are planktonic. They adults are tailless and barrel shaped. The oral and atrial endings are present at opposite ends of the tunicate. The muscular – tractions of the body wall produce water currents.

B. Subphylum Cephalochordata
Their body is laterally compressed. Body is transparent and fishlike. All four chordate characteristics persist in them throughout life. Example: Amphioxus. It has about 45 species.

C. Subphylum Vertebrata

Notochord, nerve cord, postanal tail and pharyngeal slits are present at least in embryonic stages. The vertebrae surround the nerve cord. Vertebral column acts a primary axial support. Theirskeleton is modified anteriorly into a skull. Skull protects the brain. It has eight classes.

1. Class Cephalaspidomorphi: Their body is fishlike. They are jawless. They have no paired appendages. They have cartilaginous skeleton. They have sticking mouth with teeth and rasping tongue. Example: Lampreys.

2. Class Myxini: Their body is fish like. They are jawless. They have no paired appendages. Their mouth has four pairs of tentacles. Their olfactory sacs open to mouth cavity. They have 5 to 15 pairs of pharyngeal slits. Example: Hagfishes

3. Class Chondrichthyes: They are fishlike. The jaws are present. They have paired appendages and cartilaginous skeleton. They have no swim bladder. Examples: Skates, rays, sharks.

4. Class Osteichthyes: They have bony skeleton. They swim bladder and operculum is present. Example: Bony fishes

5. Class Amphibia: Their skin produces mucous. They possess lungs or gills. They have moist skin. Skin acts as respiratory organ. There are aquatic developmental stages in them. Larva is changed by metamorphosis into adult. Example: Frogs, toads, salamanders.

6. Class Reptilia: They have dry skin with epidermal scales. They have amniotic eggs. The y have terrestrial embryonic development. Example: Snakes, lizards, alligators.

7. Class Aves: They possess feathers. They efficiently regulate body temperature (endothermic). They have amniotic eggs. Example: All birds.

8. Class Mammalia: Their bodies are partially covered by hair. They are endothermic. The young are nursed by mammary glands. They have amniotic eggs. Mammals

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thank you for your information. i already have an answer in my assignment. this site is very helpful. keep on doing good things like this. Georgesplane November 20, 2010

@ Alchemy- If you were to name the classification of the seven major phyla of humans you would call us: Animalia [kingdom] Chordata [phylum] Mammalia [class] Primata [order] Hominidae [family] Homo [genus] Homo Sapiens [species].

A chimpanzee, for example, would have the same phylum, but different genus and species, from a human. An armadillo on the other hand would share the same kingdom, phylum and class, but would differ from the order down. GenevaMech November 20, 2010

@ Alchemy- The seven major divisions of taxa are based on a systems invented by a Swedish biologist in the 18th century. They are as follows (but note that these are the main classifications and there are many other sub classifications within each classification): Kingdom, Phylum, Class, Order, Family, Genus, and Species.

Biologists can use these classifications to show the relationship between different organisms. The more taxa two organisms have in common, the more closely they are related. Alchemy November 20, 2010

I always learn something new on this website. I would have thought that the class would have fallen just below kingdom, followed closely by phylums. Naming whether something is a plant, animal, or amphibian seems like it would have been the most logical way to classify something below the title of kingdom, but that just goes to show I am no biologist. What are the other different levels of classification in taxonomy? Now I'm curious to know how humans are specifically classified.

What is chordate ? Its definition characteristics and facts

What is chordate ? The last major group of the animal kingdom is known as Phylum for chordata, it was created by Balfour in 1880. The name of phylum is derived from tw Greek words, chorde meaning a string or cord, and ata meaning bearing.

The common characteristics feature in the form of a stiff, supporting rod like a structure along the back the notochord. Notochord is Greek word formed of two word noton means back and chorda means cord, so chordate is cord bearing animal in back.

Chordate definition is, the animal which have presence of notochord or backbone in their body structure is known as chordate.

The animal belonging to all other Phylum of animal kingdom are often termed as non chordates, “the invertebrates” since they have no notochord or backbone in their body structure.

Chordate Animals

The Phylum chordata comprise of amphibia, pisces, reptiles, aves and mammals. Phylum chordata is the largest of the deuterostome phyla, it is the highest and the most important Phylum comprising a vast variety of living and extinct animals including man himself.

Most of living animals chordate are also known as vertebrates animals such as fishes amphibians, reptiles, birds and mammals.

Chordate diversity

The chordates exhibit diversity of form physiology and habit, the number of chordate species is not large, about 49000 species are on record which are only half of living species of mollusca, chordate subphylum urochordata phylum cephalochordata claim nearly 2500 species and subphylum vertebrata include about 46500, number of species fishes are 25000, amphibian species is about 2500, and reptiles about 6,000 and birds about 9000 and mammals are about nearly 4500.

Despite their low number of species of chordate make a great contribution to the Biomass of Earth. Nearly all of them are medium to large in size. The gigantic blue whale which grows to 35 metres long and 120 ton in weight is the biggest known animal.

The chordates are not only the largest animals in the existence today but ecologically they are among the most successful in the animal kingdom. All lower chordates are marine, fishes are aquatic and higher chordates are predominantly terrestrial.

Chordate three fundamental characters

All the chordates possess three out standing unique characteristics at some stage in their life cycle,these features include:-

1) A dorsal hollow or tubular nerve cord
2) a longitudinal supporting rod like notochord
3) presence of series of pharyngeal Gill slits.

These three distinctive features which set chordates apart from all other phyla,

1) dorsal hollow nerve cord: the central nervous system of chordates is present dorsally in the body, it is in the form of longitudinal hollow or tubular nerve cord lying just above the notochord and extending lengthwise in the body.

2) rod like presence of notochord: the notochord is an elongated rod like flexible structure extending the length of the body, it is present immediately beneath the nerve cord and just above the digestive canal. It originates from the endodermal roof of the embryonic archenteron.

Presence of notochord is the Prime Diagnostic feature of the Phylum chordata which derived its name from it, it serves as a support for internal a skeleton and is not to be confused with nerve cord.

3) Pharyngeal Gill slits: in all the chordates at some stage of their life cycle a series of pair literal Gill clefts or Gill slits perforated through the pharyngeal wall of gut behind the mouth. They serves primarily for the purposes of passage of water from the pharynx to outside the bathing the gills for respiration.

In Protochordates in lower aquatic vertebrate the gill slits are functional throughout life, but in higher vertebrate they disappear or become modified in the adult with acquisition of Pulmonary respiration.

Conclusion: the above three common features appear during early embryonic life of all the chordates animal but all three features rarely persist in the adult, often they are modified or even lost in the adult stage of higher chordates and notochord disappear during development in most vertebrate while nerve cord and the pharyngeal gill slits remain in the adult.

General characters of phylum chordate

1) chordate animals are aquatic, aerial Terrestrial and all free living with no fully parasite forms

2) body small to large, bilateral symmetrical and metamerically segmented

3) postanal tail is usually projects beyond the anus at some stage in may or may not persist in the adult.

4) body wall of chordates is triploblastic with three germ layers: ectoderm, mesoderm and endoderm.

5) Exoskeleton often present in chordate well developed in most vertebrate

6) Coelomate animals having a true Coelom, and enterocoelic or schizocoelic in origin.

7) a skeletal rod, the notochord present at some stage in life cycle

8) endoskeleton consists of cartilaginous or Bony, living and jointed present in the majority of members of vertebrates

9) Pharyngeal Gill slits present at some stage may or may not be functional

10) digestive system complete with digestive glands

11) blood vascular system closed type and heart ventral with dorsal and ventral blood vessels, Hepatic portal system well developed.

12) excretory system comprising proto or meso or meta nephric kidneys

Backbone in chordate animals

Origin and history of chordate

There is great deal is known about modern chordates including the lower forms their origin remain of obscure. Most zoologist now favour the deuterostome line of chordate evolution, according to which Phylum echinodermata, Hemichordata and chordate show common ancestry on embryological and biochemical evidence.


1.Describe evolutionary perspective of phylum Hemichordata.
2. Write phylogenetic relationships of Hemichordates, Echinoderms and Chordates.
3. Give general characteristics of phylum Hemichordata.
4. Describe classification of phylum chordata up to class.
5. Give classification of Chordates up to sub-phylum.
6. Give classes of sub-phylum vertebrata.
7. Discuss further phylogenetie classification of chordata.
8. How did evolution of chordates and vertebrates take place?


1. Why are tunicates called as invertebrate chordates?

Ans: Tunicates and a small group of fishlike cephalochordates lack a vertebral column.Therefore they are called the invertebrate chordates.

2. Give four basic chordate characteristics.

Ans: They have dorsal tubular nerve cord. Notochord is present in them. They have pharyngeal gill slits or gill punches. They have post anal tail.

3. Name the body parts of echinoderms.

Ans: Proboscis, collar, and trunk

4. Name the classes of Hemichordates.

Ans: Enteropneusta, Pterobranchia and Planctosphaeroidea

5. Name the subphylum of chordates.

Ans: Urochordata, Cephalochordata and Vertebrata.

6. Name the classes of sub-phylum Hemichordates.

Ans: Class Ascidiacea, Class Appendicularia (Larvacea), Class Sorberacea and Class Thaliacae

7. Name the classes of Subphylum Vertebrata.

Ans: Class Cephalaspidomorphi, Class Myxini, Class Chondrichthyes, Class

Osteichthyes, Class Amphibia, Class Reptilia, Class Ayes, Class Mammalia

8. What are two important characters of vertebrates?

Ans: Vertebral column and skull.

9. Give some examples of amphibians.

Ans: Frogs, toads, salamanders

10. Give three characteristics of class Mammalia.

Ans: Their bodies are partially covered by hair. They are endothermic. The young are nursed by mammary glands. They have amniotic eggs.

11. What are two common characteristics between chordates and


Ans: Dorsal tubular nerve cord. Pharyngeal slits of hemichordates.

12. What is paedomorphosis?

Ans: The development of sexual maturit in the larval body form is paedomorphosis.

  • Preface
  • Chapter 1. Deuterostomes and Chordates
    • 1.1. A Brief Background
    • 1.2. Deuterostomes and Chordates
    • 1.3. Deuterostome Phyla
    • 1.4. Conclusions
    • 2.1. The Annelid Theory
    • 2.2. The Auricularia Hypothesis
    • 2.3. The Calcichordate Hypothesis
    • 2.4. The Pedomorphosis Scenario: Was the Ancestor Sessile or Free-Living?
    • 2.5. The New Inversion Hypothesis
    • 2.6. The Enteropneust Hypothesis
    • 2.7. The Aboral-Dorsalization Hypothesis
    • 2.8. Conclusions
    • 3.1. The Cambrian and Ediacaran Periods
    • 3.2. Crown, Stem, and Total Groups
    • 3.3. Fossil Records of Invertebrate Deuterostomes
    • 3.4. Fossil Records of Vertebrates
    • 3.5. Conclusions
    • 4.1. Molecular Phylogeny of Metazoans
    • 4.2. Molecular Phylogeny of Deuterostome Taxa
    • 4.3. Relationships Within Each Deuterostome Phylum
    • 4.4. Xenacoelomorpha
    • 4.5. MicroRNAs
    • 4.6. Conclusions
    • 5.1. Genome Decoding
    • 5.2. Genomic Features of Five Representative Deuterostome Taxa
    • 5.3. Gene Families in Deuterostomes and the Ancestral Gene Set
    • 5.4. Exon-Intron Structures
    • 5.5. Synteny
    • 5.6. Conserved Noncoding Sequences
    • 5.7. Repetitive Elements
    • 5.8. Taxonomically Restricted Genes
    • 5.9. Conclusions
    • 6.1. Evaluation of Hypotheses for Chordate Origins
    • 6.2. The Pharyngeal Gene Cluster and the Origin of Deuterostomes
    • 6.3. Hox and Chordate Evolution
    • 6.4. ParaHox Genes
    • 6.5. Conclusions
    • 7.1. Chordate Features
    • 7.2. The New Organizers Hypothesis of Chordate Origins
    • 7.3. Cephalochordate Embryogenesis: Primitive Chordate Body-Plan Formation
    • 7.4. Chordate Features and Molecular Developmental Mechanisms
    • 7.5. The Notochord: A Mesodermal Novelty
    • 7.6. Somites (Myotomes): A Mesodermal Novelty
    • 7.7. The Postanal Tail: A Mesodermal Novelty
    • 7.8. The Dorsal Central Nervous System: An Ectodermal Novelty
    • 7.9. Hatschek’s Pit: An Ectodermal Novelty
    • 7.10. The Endostyle: An Endodermal Novelty
    • 7.11. Conclusions
    • 8.1. Spemann’s Organizer, the Nieuwkoop Center, and the Three-Signal Model
    • 8.2. Axial Patterning of Deuterostome Body Plans
    • 8.3. Interpretation of the Dorsoventral-Axis Inversion Hypothesis
    • 8.4. Conclusions
    • 9.1. The Stomochord and Other Organs Proposed as Antecedents to the Notochord
    • 9.2. The Nervous System of Enteropneusts
    • 9.3. The Spemann’s Organizer-Like System in Hemichordates
    • 9.4. Interpretations of the Enteropneust Hypothesis
    • 9.5. Conclusions
    • 10.1. Evolution of Vertebrates
    • 10.2. Evolution of Urochordates
    • 10.3. Conclusion
    • 11.1. The Three-Phylum System of Chordates
    • 11.2. Mechanisms Involved in Origination of Deuterostome Novelties
    • 11.3. Horizontal Gene Transfer
    • 11.4. The Significance of Gene Duplication in Deuterostome Evolution
    • 11.5. Significance of Domain Shuffling in Chordate Evolution
    • 11.6. The Significance of Structural Genes in Metazoan Evolution
    • 11.7. The Phylotypic Stage
    • 11.8. Conclusions
    • 12.1. Summary
    • 12.2. Perspective

    Noriyuki Satoh

    Noriyuki Satoh is a Professor of the Marine Genomics Unit, Okinawa Institute of Science and Technology, Graduate University, Okinawa, Japan. After obtaining a PhD at the University of Tokyo, he carried out research of developmental biology of tunicates at Kyoto University. Satoh and his colleagues have established Ciona intestinalis as a model organism of developmental biology, and he has also conducted research of developmental mechanisms involved in the origins and evolution of chordates. Dr. Satoh’s group has also disclosed molecular mechanisms of notochord formation, and he is one of the leaders of the genome decoding projects of marine invertebrates, including tunicates, cephalochordates, and hemichordates.

    Affiliations and Expertise

    Department of Zoology, Graduate School of Science, Kyoto University, Kyoto, Japan

    Watch the video: Introduction To Chordata. Diversity In Living Organisms. Biology. Class 9 (November 2021).