BSc 2nd Year Zoology Phylum Chordata Notes Study Material

BSc 2nd Year Zoology Phylum Chordata Notes Study Material

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BSc 2nd Year Zoology Phylum Chordata Notes Study Material
BSc 2nd Year Zoology Phylum Chordata Notes Study Material

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 recognized. The last major group of the Animal Kingdom is known as the 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. (BSc 2nd Year Zoology Phylum Chordata Notes Study Material)

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 the man himself. Most of the living chordates are well-known familiar vertebrate animals such as fishes, amphibians, reptiles, birds, and mammals. Besides, they include a number of marine forms such as tunicates and lancelets that are less well known.

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 is only half of the living species of molluscs, and less than one-tenth of those of arthropods. The two subphyla Urochordata and Cephalochordata claim 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.

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 35 meters long and 120 tons in weight, is the biggest known animal. The whale shark (Rhincodon 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 Notes Study Material)

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 climates because they have a constant body temperature, something no other animals have. (BSc 2nd Year 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 water. 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 the 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. (BSc 2nd Year Zoology Phylum Chordata Notes Study Material)

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 of even body activities.

BSc 2nd Year Zoology Phylum Chordata Study Material Notes
T.S. Notochord of young dogfish.

The posterior part of the nerve cord becomes the spinal cord and is 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. Protochordate has 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 pharyngeal, branchial and visceral clefts or pouches. They serve primarily for the passage of water from the pharynx to the outside, thus bathing the gills for respiration. The water current secondarily aids in filter-feeding by retaining food particles in the pharynx.

In protochordate (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 the early embryonic life of all the chordates. But all 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 2nd Year 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. (BSc 2nd Year Zoology Phylum Chordata Notes Study Material)

Characters Common to Chordates and Higher Non-chordates

There are a number of features in which the chordates resemble the higher non chordates or invertebrates.

1. Aviation: The body in both has a distinct polar axis. The anterior end is differentiated into a cephalic region or head that usually precedes locomotion. The opposite or posterior forms a tail in most cases. This longitudinal axis of the body running from head to tail is known as the anteroposterior axis. However, this axial 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 non chordates (Protostomia), but forms the anus in chordates (Deuterostomia).

2. Bilateral symmetry: Due to the existence of a longitudinal anteroposterior axis, the body of all chordates and most higher non chordates (Annelida, Arthropoda) exhibits bilateral symmetry, i.e., the right and left sides of the body are the exact mirror image of each other.

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.

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 non chordates. 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.

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 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 the 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.

These structural similarities shared by the chordates and the non chordates probably reflect upon their remote common ancestry. Although it is not possible to trace exact lineage, all available evidence indicates that chordates have evolved from invertebrates.

Advancements of Chordata over Other Phyla

Phylum Chordata has some advantages over other phyla due to certain characteristics.

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 non-chordate phyla. Further, this living endoskeleton permits greater freedom of movement and indefinite growth so that many chordates are the largest creatures in the animal world. (BSc 2nd Year Zoology Phylum Chordata Notes Study Material)

2. Efficient respiration: The gills in aquatic chordates and the lungs in terrestrial form efficient organs of respiration. The tracheal system of certain arthropods is also efficient but is suitable for 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.

4. Centralized nervous system: The invertebrate phyla show a growing tendency of centralization of the nervous system, reaching its culmination in the higher chordates. (BSc 2nd Year Zoology Phylum Chordata Notes Study Material)

Differences between 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 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 practical applicability in their taxonomic study.

We have also seen that chordates share many structural similarities with the non chordates 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.

BSc 2nd Year Zoology Phylum Chordata Study Material Notes
Differences of Chordata with Nonchordata.

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 the Cambrian period as the oldest fossils of known vertebrates have been discovered in the late Cambrian strata. Most scientists consider that the chordates originated from invertebrates.

Several theories attempt to explain the origin of chordates from non-chordate groups, but they have serious drawbacks and are far from satisfactory. One theory advocates the descent of Chordata from the Echinodermata as such. The remarkable similarities between the echinoderm (bipinnaria) and hemichordate (tornaria) larvae are taken as good evidence for common ancestry.

Most zoologists (Romer, Becerril, Barrington, etc.) now favor the deuterostome line of chordate evolution, according to which the phyla Echinodermata, Hemichordata, and Chordata show common ancestry on embryological and biochemical evidence. The protochordate provides the connecting link between early chordate ancestors and vertebrates. The differentiation probably occurred much earlier than the Cambrian period. The earliest traces of vertebrates have been found in the rocks of the 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. (BSc 2nd Year Zoology Phylum Chordata Notes Study Material)

Major Subdivisions of Phylum Chordata

Phylum Chordata is a rather heterogeneous assemblage of groups that differ widely from one another and show various degrees of relationships with 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.

1. Subphyla and classes: Phylum Chordata is first conveniently separated into 3 or 4 primary subdivisions, called subphyla, based on the character of 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., vertebrates, 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
Subdivisions of the phylum chordata

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 to 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 they are known as Acrania (Gr., an 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 gnathostomes 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 recognizes 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 are free-living with no fully parasitic forms.

(2) Body is 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.

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

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

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

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

BSc 2nd Year Zoology Phylum Chordata Study Material Notes
Outline Classification of Phylum Chordata

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

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

(10) Digestive system is complete with digestive glands.

(11) Blood vascular systems closed. Heart ventral with dorsal and ventral blood vessels. The hepatic portal system is well-developed.

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

(13) Nerve cord dorsal and tubular. The anterior end is usually enlarged to form the brain.

(14) Sexes separate with rare exceptions.

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. (BSc 2nd Year Zoology Phylum Chordata Notes Study Material)


(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, a 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. (BSc 2nd Year Zoology Phylum Chordata 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 a brain. About 2,000 species.

The Acrania is divided into three subphyla: Hemichordata, Urochordata, and Cephalochordata.

Subphylum I. Hemichordata

(Gr. Hemi, half; chorde, cord). The body is divided into 3 regions: proboscis, collar, and trunk. Notochord doubtful, short, confined to the proboscis, and non-homologous with that of chordates.

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 include 2 orders, 3 genera, and 20 species. Cephalodiscus, Rhabdopleura.

Class 3. Planctosphaeroidea: Transparent, round, and specialized tornaria larva, having extensively branched ciliary bands and an L-shaped alimentary canal, represents this class. Planctosphaera pelagic. This form is regarded as the larval form of some unknown hemichordates. (BSc 2nd Year Zoology Phylum Chordata Notes Study Material)

Class 4. Graptolite: The fossil graptolites (e.g. Dendrograptus) were abundant in the Ordovician and Silurian periods and often placed as an extinct class under Hemichordata. Their tubular chitinous skeleton and colonial habits show an affinity with Rhabdopleura.

Subphylum II. Urochordata or Tunicata

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

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

Contrasting characters of Protochordata and Euchordata.
Contrasting characters of Protochordata and Euchordata.

Class 2. Thaliacea: Free-swimming or pelagic tunicates with circular muscles in the 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 the tail. Only two-gill slits. 2 orders, 2 families, 5 genera, and 30 species. Oikopleura.

Subphylum III. Cephalochordata

(Gr. kephale, head; L. chorda, cord). Notochords and nerve cords are present throughout life along the entire length of the body.

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 heads, vertebral columns, jaws, and brains 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.

Class 1. Ostracodermi: (Gr. ostrakon, sheli; derma, skin). Several extinct orders of ancient primitive heavily armored, Palaeozoic, the 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).

Division II. Gnathostomata

(Gr. gnathos, jaw; stoma, mouth). Jawed vertebrates have true jaws and paired limbs.

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

Superclass 1. Pisces

(L. piscis, fish). Fishes of fish-like aquatic forms with paired as well as median fins, gills, and scaly skin.

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 claspers in males. Cartilaginous fishes. Approximately 600 species. Scoliodon (dogfish), Chimaera (ratfish). (BSc 2nd Year Zoology Phylum Chordata Notes Study Material)

Class 3. Osteichthyes: (Gr. osteon, bone; ichthys, fish). Freshwater and marine. Endoskeleton is mostly bony. The skin has 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 2nd Year 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.

Class 1. Amphibia: (Gr. amphi, both; bios, life). The larval stage is usually aquatic and breathes by gills. Adults are typically terrestrial and respire by lungs. Skin moist, glandular, and with no external scales. Heart 3-chambered. Amphibians. Approximately 2,500 species. Rana (frog), Bufo (toad) Ambystoma (salamander). (BSc 2nd Year Zoology Phylum Chordata Notes Study Material)

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). (BSc 2nd Year Zoology Phylum Chordata Notes Study Material)

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

Class 4. Mammalia: (L. mamma, breast). Body covered by hair. Skin glandular. Females with mammary glands 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)

BSc 2nd Year Zoology Phylum Chordata Notes Study Material

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