BSc 2nd Year Zoology Phylum Chordata Notes Study Material

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BSc 2nd Year Zoology Phylum Chordata Notes Study Material

What is Chordata?

In the scheme of classification, the Animal Kingdom is divided first into several major animal groups called phyla (singular, phylum). There are approximately 30 animal phyla currently recognised. The last major group of the Animal Kingdom is known as phylum Chordata. It was created by Balfour in 1880. The name of this phylum is derived from two Greek words, the chorde meaning a string or cord, and ata meaning bearing.

Thus, chordates are animals having a cord, i.e., notochord. The animals belonging to all other phyla of the Animal Kingdom are often termed ‘the non-chordates’ or ‘the invertebrates’ since they have no notochord or backbone in their body structure.

Chordate Animals

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 the living chordates are the well known familiar vertebrate animals such as the fishes, amphibians, reptiles, birds and mammals. Besides, they include a number of marine forms such as the tunicates and lancelets that are less well known.(Phylum Chordata Notes Study Material)

Diversity of Chordates

The chordates exhibit an astonishing diversity of form, physiology and habit.

Numerical strength: The number of chordate species is not unusually large. About 49,000 species are on record which are only half of the living species of molluscs, and less than one-tenth those of arthropods. The two subphyla Urochordata in and Cephalochordata claims for nearly 2,500 species. The subphylum Vertebrata includes 46,500 species; of these, fishes are the most numerous with approximately 25,000 species. It is commonly assumed that amphibian species number about = 2,500, reptiles 6,000, birds 9,000 and mammals 4,500.(BSc Zoology Phylum Chordata Notes Study Material)

Size: Despite their modest number of species, the chordates make a disproportionate contribution to the biomass of the earth. Nearly all of them are medium to large in size. The vertebrates, in particular, are considerably large and many of them are among the largest of living animals. The gigantic blue whale (Balaenoptera musculus), which grows to 35 meters long and 120 tons in weight, is the biggest known animal. The whale shark (Rhineodon typus) reaches a length of 15 meters and is the second biggest vertebrate after whales. The smallest fish is a Philippine goby (Pandaka) measuring only 10 mm long.

BSc 2nd Year Zoology Phylum Chordata Study Material Notes

Ecology: The chordates are not only the largest animals in existence today, but ecologically they are among the most successful in the Animal Kingdom. They are able to occupy most kinds of habitats and they have adapted themselves to more modes of existence than any other group, including the arthropods. They are found in the sea, in freshwater, in the air, and on all parts of land from the poles to the equator. Birds and mammals have been able to penetrate cold climate because they have a constant body temperature, something no other animals have.(BSc Zoology Phylum Chordata Notes Study Material)

All lower chordates are marine, fishes are aquatic and higher chordates are predominantly terrestrial. Saltwater amphibians are unknown. No bird lives permanently in water, while some amphibians, reptiles and mammals occur as permanent residents of waters. While most tunicates are sessile, all chordates are free-living and none is strictly parasitic.(Zoology Notes Study Material)

The chordates are probably the most conspicuous and the best-known group in the entire animal kingdom, partly because of their large size and partly because of the important role they perform in their ecosystems. They are of primary interest because man himself is a member of the group. From a purely biological viewpoint also the chordates are interesting because they illustrate ‘so well the broad biological principles of evolution, development and relationship.

Three Fundamental Chordate Characters

All the chordates possess three outstanding unique characteristics at some stage in their life history. These three fundamental morphological features include:

(1) A dorsal hollow or tubular nerve cord

(2) A longitudinal supporting rod-like notochord

(3) A series of pharyngeal gill slits

These three distinctive features, which set chordates apart from all other phyla, are so important that each merits a short description of its own.

1. Dorsal hollow nerve cord: The central nervous system of the chordates is present dorsally in the body. It is in the form of a longitudinal hollow or tubular nerve cord lying just above the notochord and extending lengthwise in the body. There are no distinct ganglionic enlargements. The nerve cord serves for the integration and coordination even the body activities.

The posterior part of nerve cord becomes the spinal cord and protected within the vertebral column.

2. Notochord or chorda dorsalis: 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. Structurally, it is composed of large vacuolated notochordal cells containing a gelatinous matrix and surrounded by an outer fibrous and an inner elastic sheath.

BSc 2nd Year Zoology Phylum Chordata Study Material Notes

The notochord is the prime diagnostic feature of the phylum Chordata which derives its name from it. It serves as a support or internal skeleton and is not to be confused with the nerve cord. Protochordates have a typical notochord. In adult vertebrates, it is surrounded or replaced by the vertebral column.

3. Pharyngeal gill slits: In all the chordates, at some stage of their life history, a series of paired lateral gill clefts or gill slits perforate through the pharyngeal wall of the gut behind the mouth. These are variously termed as pharyngeal, branchial and visceral clefts or pouches. They serve primarily for the passage of water from the pharynx to outside, thus bathing the gills for respiration. The water current secondarily aids in filter feeding by retaining food particles in the pharynx.

In protochordates (e.g. Branchiostoma) and lower aquatic vertebrates the gill slits are functional throughout life. But, in higher vertebrates, they disappear or become modified in the adult with the acquisition of pulmonary respiration.

The above three common features appear during early embryonic life of all the chordates. But all the three features rarely persist in the adult (e.g. Branchiostoma). The notochord disappears during development in most vertebrates, while the nerve cord and the pharyngeal clefts or their derivatives remain in the adult.(BSc Zoology Phylum Chordata Notes Study Material)

The three common chordate characters were probably characteristic of the ancestral chordates. They distinguish chordates from all other animals and appear to reveal their common ancestry.

Characters Common to Chordates and Higher Non-chordates

There are a number of features in which the chordates resemble the higher nonchordates or invertebrates. (Phylum Chordata Notes Study Material)

1. Axiation: The body in both has a distinct polar axis. The anterior end is differentiated into a cephalic region or head that usually precedes in locomotion. The opposite or posterior and forms a tail in most cases. This longitudinal axis of the body running from head to tail is known as the antero-posterior axis. However, this axiate body organization is not strictly homologous in the two groups. For instance, the head in the two groups is not homologous because the blastoporal end develops the mouth in most nonchordates (Protostomia), but forms the anus in chordates (Deuterostomia). (Zoology Phylum Chordata Study Material Notes)

2. Bilateral symmetry: Due to existence of longitudinal antero-posterior axis, the body of all chordates and most higher nonchordates (Annelida, Arthropoda) exhibits bilateral symmetry, i.e., the right and left sides of the body are the exact mirror image of each other.(BSc Zoology Phylum Chordata Notes Study Material)

3. Triploblastic condition: Invertebrates above the level of coelenterates and all chordates are triploblastic animals. They have three germ layers-ectoderm, endoderm and mesoderm. In annelids, molluscs, arthropods and other related animals, the embryonic mesoderm arises as a solid cord-like outgrowth from the junction of ectoderm and endoderm in the gastrula stage.

BSc 2nd Year Zoology Phylum Chordata Study Material Notes

1. Coelom: A secondary body cavity or true coelom exists between the body wall and the digestive tube, and it is lined on all sides by mesoderm. However, it differs in its mode of origin in different groups of chordates and nonchordates. In annelids, molluscs, arthropods and higher chordates, it is schizocoelous, arising from the splitting of mesodermal bands which originate from the blastoporal region, In echinoderms, brachiopods and Branchiostoma, it is enterocoelous, formed by the fusion and expansion of a linear series of hollow outpushings or pouches of the archenteron.(Zoology Phylum Chordata Notes Study Material)

2. Metamerism: Metamerism is a condition in which the body is composed of a linear series of similar body segments, called metameres or somites. It is found in three phyla Annelida, Arthropoda and Chordata. In Annelida and Arthropoda, metameric segmentation is both the internal as well as external, whereas in Chordata is it less clear externally. External segmentation is a necessity in Arthropoda so that joints between segments permit movement of the body. In fishes, metameric segmentation is visible in the arrangement of muscles.

3. Organ-systems: In an organ-system, several organs work together for the same function, such as digestion, circulation, respiration, etc. Organ-system plan or grade of organization is shown by all the chordates and all the non-chordates from nemertean worms onwards. However, the vertebrates show a greater stage of development and fundamental unity of this plan than even the highest invertebrates.(BSc Zoology Phylum Chordata Notes Study Material)

These structural similarities shared by the chordates and the nonchordates probably reflect upon their remote common ancestry. Although it is not possible to trace exact lineage, but all available evidences indicates that chordates have evolved from the invertebrates.(Zoology Phylum Chordata Notes Study Material)

Advancements of Chordata over Other Phyla

Phylum Chordata has some advantages over other phyla due to certain characters.(Zoology Notes Study Material)

1. Living endoskeleton: With the exception of echinoderms and a few others, only chordates possess a living endoskeleton. It grows in size with the rest of the body so that there is no compulsion to shed it periodically to allow for growth like the nonliving chitinous exoskeleton of the nonchordate phyla. Further, this living endoskeleton permits greater freedom of movement and indefinite growth so that many chordates are the largest creatures of animal world.

2. Efficient respiration: The gills in aquatic chordates and the lungs in terrestrial forms from efficient organs of respiration. The tracheal system of certain arthropods is also efficient but is suitable to animals of small size only.

3. Efficient circulation: The circulatory system of the chordates is well developed and the blood flows freely in the respiratory organs ensuring rapid exchange of gases. Moreover, the blood-vascular system forms an important medium for several other vital activities of the body. (Zoology Phylum Chordata)

4. Centralized nervous system: The invertebrate phyla show a growing tendency of centralization of nervous system, reaching its culmination in the higher chordates.

Differences of Chordates with Non-chordates

So far we have seen that the Animal Kingdom is traditionally divided into two unequal groups: Chordates and Non-chordates (or Invertebrates). All 20 the chordates belong to a single phylum, the Chordata, which are characterized by the presence of a dorsal hollow nerve cord, a series of pharyngeal gill clefts and a unique supporting axial rod, the notochord or vertebral column running through the length of the body. The non-chordates or invertebrates belong to the remaining 30 or odd phyla of animals that do not possess a notochord or vertebral column. This division is purely artificial but this has a practical applicability in their taxonomic study. (BSc Zoology Phylum Chordata Notes Study Material)

We have also seen that chordates share many structural similarities with the nonchordates such as axiate body plan, bilateral symmetry, triploblastic coelomate condition, organ systems, metameric segmentation, etc. However, the two groups have several fundamental differences, the most striking of which have been shown.

Origin and Ancestry of Chordata

While a great deal is known about modern chordates, including the lower forms, their origin remains obscure. Scientists have not succeeded in determining which lower forms have given rise to them. Their early ancestors most likely were soft-bodied and left no definite fossil remains. They must have originated prior to Cambrian period as the oldest fossils of known vertebrates have been discovered in late Cambrian strata. Most scientists consider that the chordates have originated from invertebrates. Several theories attempt to explain the origin of chordates from nonchordate groups, but they have serious drawbacks and are far from being satisfactory. One theory advocates the descent of Chordata from the Echinodermata as such. The remarkable similarities between the echinoderm (bipinnaria) and hemichordate (tornaria) larvae is taken as good evidence for common ancestry.(BSc Zoology Phylum Chordata Notes Study Material)

Most zoologists (Romer, Berril, Barrington, etc.) now favour the deuterostome line of chordate evolution, according to which the phyla Echinodermata, Hemichordata and Chordata show common ancestry on embryological and biochemical evidences. The protochordates provide the connecting link between early chordate ancestors and verebrates. The differentiation probably occurred much earlier than Cambrian period. The earliest traces of vertebrates have been found in the rocks of late Cambrian and Ordovician. A number of fishes followed in Silurian and became abundant in the Devonian. The subsequent periods show the evolution of amphibians, reptiles, birds and mammals.

Major Subdivisions of Phylum Chordata

Phylum Chordata is a rather heterogeneous assemblage of groups which differ widely from one another and show various degrees of relationships to each other. In a taxonomic outline or classification of the phylum, these groups are customarily arranged in larger functional divisions or subdivisions based on specific structures or features. These subdivisions or taxa have been accorded different ranks under different systems of classification. The following terminology includes major subdivisions of phylum Chordata.(Zoology Notes Study Material)

1. Subphyla and classes: Phylum Chordata is first conveniently separated into 3 or 4 primary subdivisions, called subphyla, based on the character on notochord. These are:

Subphylum 1. Hemichordata (or Adelochordata)

Subphylum 2. Urochordata (or Tunicata)

Subphylum 3. Cephalochordata

Subphylum 4. Vertebrata

Subphylum Hemichordata (Gr., hemi, half; chorde, cord) have long been traditionally considered to be the lowest chordates. But recent workers consider the so-called notochord of the hemichordates not a true notochord but a ‘stomochord’.

Subphylum Urochordata (Gr., oura, a tail; L., chorda, cord) includes 3 classes (Larvacea, Ascidiacea and Thaliacea), subphylum Cephalochordata (Gr., kephale, head; chorde, cord) includes a single class (Leptocardii), while the subphylum Vertebrata (L., vertebratus, backbone) is subdivided into 9 classes (Ostracodermi, Cyclostomata, Placodermi, Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves and Mammalia).(Zoology Notes Study Material)

BSc 2nd Year Zoology Phylum Chordata Study Material Notes

2. Protochordata and Euchordata: The first two subphyla under phylum Chordata (i.e., Urochordata and Cephalochordata) are all marine. relatively small and without a vertebral column or backbone. They are often collectively referred as the nonvertebrate or invertebrate chordates or protochordates, (Gr., protos, first; chorde, cord), as they are regarded to be early, primitive, borderline or first chordates closely allied with the ancestral chordate stock.

Therefore, the term ‘Protochordata’ no longer holds any official taxonomic position, but is merely descriptive. Its use does not reflect any close relationship between the two or three subphyla, but simply contrasts their general position with the greatly advanced vertebrates.(Zoology Notes Study Material)

The third subphylum, i.e., Vertebrata, provided with a vertebral column, is regarded to be more advanced and belongs to the subdivision Euchordata of the phylum Chordata.

3. Acrania and Craniata: The protochordate subphyla lack a head and a cranium, so that they are known as Acrania (Gr., a absent; kranion, head). On the other hand, the subphylum Vertebrata (subdivision Euchordata) has a distinct head and a cranium and is also called Craniata. Some workers consider Acrania synonymous with the subphylum Cephalochordata alone.

BSc 2nd Year Zoology Phylum Chordata Study Material Notes

4. Agnatha and Gnathostomata: The Vertebrata (or Craniata) are further subdivided in several ways. One possibility with universal agreement separates them into two unequal sections: Agnatha and Gnathostomata. Agnatha (Gr., a, not; gnathos, jaw) lack true jaws and paired appendages. Agnathans or agnathostomes include a small number of primitive but highly specialized fish-like forms, the extinct ostracoderms and the modern cyclostomes. Some workers include even the protochordates in Agnatha. All other vertebrates have true jaws and paired appendages, and are called Gnathostomata (Gr., gnathos, jaw; stoma, mouth).

5. Pisces and Tetrapoda: A basic division of Gnathostomata recognises two superclasses: Pisces and Tetrapoda. The superclass Pisces (L., piscis, fish) includes all the fishes which are strictly aquatic forms with paired fins. (Zoology Notes Study Material)

6. Anamnia and Amniota: Another method of grouping the vertebrates elicits an extremely important advance in the pattern of embryological development. It is based on the presence of a special membrane, the amnion, that holds the developing embryo in a reservoir of fluid, and permits the laying of eggs on land. The animals that possess it belong to Amniota which includes the classes Reptilia, Aves and Mammalia. The animals without this membrane are Anamniota including cyclostomes, fishes and amphibians.

General Characters of Phylum Chordata

(1) Aquatic, aerial or terrestrial. All free-living with no fully parasitic forms.

(2) Body small to large, bilaterally symmetrical and metamerically segmented.

(3) A postanal tail usually projects beyond the anus at some stage and may or may not  persist in the adult.(Zoology Notes Study Material)

(4) Exoskeleton often present; well developed in most vertebrates.

(5) Bodywall triploblastic with 3 germinal layers : ectoderm, mesoderm and endoderm.(Zoology Notes Study Material)

(6) Coelomate animals having a true coelom, enterocoelic or schizocoelic in origin.(Zoology Notes Study Material)

(7) A skeletal rod, the notochord, present at some stage in life cycle.

(8) A cartilaginous or bony, living and jointed endoskeleton present in the majority of members (vertebrates).(Zoology Notes Study Material)

(9) Pharyngeol gill slits present at some stage; may or may not be functional.

(10) Digestive system complete with digestive glands.

(11) Blood vascular systems closed. Heart ventral with dorsal and ventral blood vessels. Hepatic portal system well developed.(Zoology Notes Study Material)

(12) Excretory system comprising proto-or meso-or meta-nephric kidneys.

(13) Nerve cord dorsal and tubular. Anterior end usually enlarged to form brain.

(14) Sexes separate with rare exceptions.(Zoology Notes Study Material)

Brief Classification of Chordata with Characters

The chordates form a large heterogeneous grouping of members differing widely from one another in many respects. This creates problems in their systematic classification. Table provides an outline classification of the phylum Chordata.

PHYLUM CHORDATA

(L. chordatus, having a cord, i.e., the notochord). Widely diversified in size, habits and habitat. Bilaterally symmetrical, metamerically segmented, triploblastic, coelomate deuterostomes. All possess, at some stage in their life-history, supporting skeletal rod or notochord, a hollow dorsal nerve cord and paired gills or gill-slits, which may persist, change or disappear in the adults. Cambrian to Recent Chordates. About 50,000 species.(Zoology Notes Study Material)

Phylum Chordata can be divided into two groups:- Acrania (Protochordata) and Craniata (Euchordata) having contrasting characters.

Group A. Acrania (Protochordata)

(Gr. a, absent; kranion, head, or, Gr. protos, first; chorde, cord). All marine, small. primitive or lower chordates. Lacking a head, a skull or cranium, a vertebral column, jaws and brain. About 2,000 species.(Zoology Notes Study Material)

The Acrania is divided into three subphyla: Hemichordata, Urochordata and Cephalochordata.(Zoology Notes Study Material)

Subphylum I. Hemichordata

(Gr. Hemi, half; chorde, cord). Body divided into 3 regions: proboscis, collar and trunk. Notochord doubtful, short, confined to proboscis and non-homologous with that of chordates.(BSc Zoology Phylum Chordata Notes Study Material)

Class 1. Enteropneusta: (Gr. enteron, gut; pneustos, breathed). Body large and worm-like. Gill-slits numerous. Intestine straight. Acorn or tongue worms. 3 families, 15 genera and 70 species. Balanoglossus, Saccoglossus.

Class 2. Pterobranchia: (Gr. pteron, feather; branchion, gill). Body small and compact. Gill-slits one pair or none, Intestine U-shaped. Pterobranchs includes 2 orders, 3 genera and 20 species. Cephalodiscus, Rhabdopleura.

Class 3. Planctosphaeroidea: Transparent, round and specialized tornaria larva, having extensively branched ciliary bands and L-shaped alimentary canal, represents this class. Planctosphaera pelagica. This form is regarded as the larval form of some unknown hemichordates.(Zoology Notes Study Material)

Class 4. Graptolita: The fossil graptolites (e.g. Dendrograptus) were abundant in Ordovician and Silurian periods and often placed as an extinct class under Hemichordata. Their tubular chitinous skeleton and colonial habits shoe an affinity with Rhabdopleura.(BSc Zoology Phylum Chordata Notes Study Material)

Subphylum II. Urochordata or Tunicata

(Gr. oura, a tail; L. chorda, cord). Notochord and nerve cord only in tadpole-like larva. Adult sac-like, often sessile and encased in a protective tunic. Tunicates.

Class 1. Ascidiacea: Sessile tunicates with scattered muscles in tunic. Solitary, colonial or compound. Gill-clefts numerous. Ascidians or sea squirts. 2 subclass, 3 orders, 12 families, 37 genera and 1,200 species. Herdmania, Ciona, Molgula.

Class 2. Thaliacea: Free-swimming or pelagic tunicates with circular muscles in tunic. Sometimes colonial. Salps or chain tunicates. 3 orders, 5 families, 9 genera and 30 species. Salpa, Doliolum, Pyrosoma.

Class 3. Larvacea or Appendicularia: Tiny, transparent, free-floating. Adults retain many larval features including tail. Only two gill-slits. 2 orders, 2 families, 5 genera and 30 species. Oikopleura.

Subphylum III. Cephalochordata

(Gr. kephale, head; L. chorda, cord). Notochord and nerve cord present throughout life along entire length of body.(Zoology Notes Study Material)

Class Leptocardii: Body fish-like, segmented with distinct myotomes and numerous gill-slits. Free-swimming and burrowing. Lancelets. One class, one family, 2 genera and 30 species. Branchiostoma (=Amphioxus), Asymmetron. About eight species have been recognized under the genus Branchiostoma & six species have been recognized from the genus Asymmetron.

Group B. Craniata (Euchordata)

Aquatic or terrestrial, usually large-sized, higher chordates or vertebrates with distinct head, a vertebral column, jaws and brain protected by a skull or cranium. The Craniata includes a single subphylum, the Vertebrata.

Subphylum IV. Vertebrata

(L. vertebratus, backbone). Notochord supplemented or replaced by a vertebral column or backbone composed of overlapping vertebrae. Body divisible into head, neck, trunk and tail. Usually dioecious. Vertebrates. Largest chordate subphylum including about 46,500 species.

The subphylum Vertebrata is divided into two divisions: Agnatha & Gnathostomata characters as follows:

Division I. Agnatha

(Gr. a, not; gnainos, jaw). Jawless primitive fish-like vertebrates without true jaws and paired limbs.(BSc Zoology Phylum Chordata Notes Study Material)

Class 1. Ostracodermi: (Gr. ostrakon, sheli; derma, skin). Several extinct orders of ancient primitive heavily armoured, Palaeozoic, world’s first vertebrates, collectively called the ostracoderms. Cephalaspis, Drepanaspis.

Class 2. Cyclostomata: (Gr. cyklos, circular; stoma, mouth). Body eel-shaped, without scales, jaws and lateral fins. Mouth rounded and suctorial. Gills 5-16 pairs. Parasites and scavengers. 45 species. Lampreys (Petromyzon) and hagfishes (Myxine).(BSc Zoology Phylum Chordata Notes Study Material)

Division II. Gnathostomata

(Gr. gnathos, jaw; stoma, mouth). Jawed vertebrates having true jaws and paired limbs.(BSc Zoology Phylum Chordata Notes Study Material)

For convenience, some taxonomists further divide Gnathostomata into two superclasses. Their contrasting features are as follows:

Superclass 1. Pisces

(L. piscis, fish). Fishes or fish-like aquatic forms with paired as well as median fins, gills and scaly skin.(BSc Zoology Phylum Chordata Notes Study Material)

Class 1. Placodermi: Several extinct orders of primitive earliest jawed fishes of Palacozoic with bony head shield movably articulated with trunk shield. Placoderms. Climatius, Dinichthys.(Zoology Notes Study Material)

Class 2. Chondrichthyes: (Gr. chondros, cartilage; ichthys, fish). Mostly marine. Cartilaginous endoskeleton. Skin with placoid scales. Gill-slits not covered by operculum. Pelvic ciaspers in male. Cartilaginous fishes. Approximately 600 species. Scoliodon (dogfish), Chimaera (ratfish).

Class 3. Osteichthyes: (Gr. osteon, bone; ichthys, fish). Freshwater and marine. Endoskeleton mostly bony. Skin having various types of scales (cycloid, ctenoid) other than placoid. Gill-slits covered by operculum. Males without claspers. Bony fishes. 20,000 species. Labeo (rohu). Protopterus (lungfish), Hippocampus (sea horse).(BSc Zoology Phylum Chordata Notes Study Material)

Superclass 2. Tetrapoda

(Gr. tetra, four; podos, foot). Land vertebrates with two pairs of pentadactyle limbs, cornified skin and lungs.(Zoology Notes Study Material)

Class 1. Amphibia: (Gr. amphi, both; bios, life). Larval stage usually aquatic and breathes by gills. Adult typically terrestrial and respires by lungs. Skin moist, glandular and with no external scales. Heart 3-chambered. Amphibians. Approximately 2,500 species. Rana (frog), Bufo (toad) Ambystoma (salamander).

Class 2. Reptilia: (L. reptilis, creeping). Terrestrial tetrapods. Skin dry, covered by ectodermal horny scales or bony plates. Heart incompletely 4-chambered. Cold-blooded. Respiration by lungs. 7,000 species. Hemidactylus (wall lizard), Uromastix (spiny-tailed lizard), Naja (cobra), Sphenodon, Crocodilus (crocodile).

Class 3. Aves: (L. avis, bird). Typically flying vertebrates covered with feathers. Forelimbs modified into wings. No teeth in beak. Heart 4-chambered. Warm blooded. Birds. About 9,000 species. Struthio (African ostrich), Columba (pigeon), Gallus (fowl).(BSc Zoology Phylum Chordata Notes Study Material)

Class 4. Mammalia: (L. mamma, breast). Body covered by hair. Skin glandular. Female with mammary glands which secrete milk for suckling the young. Heart 4-chambered. Warm blooded, air – breathing vertebrates. 4,500 species. Echidna (spiny anteater), Macropus (Kangaroo), Rattus (rat), Homo (man).

BSc 2nd Year Zoology Phylum Chordata Notes Study Material

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