Phylum  

Chordata  

includes  

animals  

which  

possess  

a  

notochord  

either

throughout   

or   

during   

early   

embryonic   

life.   

Other   

common   

features

observed  

in  

the  

chordates  

are  

the  

dorsal,  

hollow  

nerve  

cord  

and  

paired

pharyngeal  

gill  

slits.  

Some  

of  

the  

vertebrates  

do  

not  

possess  

jaws

(Agnatha)   

whereas   

most   

of   

them   

possess   

jaws   

(Gnathostomata).

Agnatha  

is  

represented  

by  

the  

class,  

Cyclostomata.  

They  

are  

the  

most

primitive  

chordates  

and  

are  

ectoparasites  

on  

fishes.  

Gnathostomata

has  

two  

super  

classes,  

Pisces  

and

Tetrapoda.  

Classes  

Chondrichthyes

and   

Osteichthyes   

bear   

fins   

for   

locomotion   

and   

are   

grouped   

under

Pisces.  

The  

Chondrichthyes  

are  

fishes  

with  

cartilaginous  

endoskeleton

and  

are  

marine.  

Classes,

Amphibia,  

Reptilia,

Aves  

and  

Mammalia  

have

two   

pairs   

of   

limbs   

and   

are   

thus   

grouped   

under   

Tetrapoda.   

The

amphibians  

have  

adapted  

to  

live  

both  

on  

land  

and  

water.  

Reptiles  

are

characterised  

by  

the  

presence  

of  

dry  

and  

cornified  

skin.  

Limbs  

are

absent  

in  

snakes.  

Fishes,  

amphibians  

and  

reptiles  

are  

poikilothermous

(cold-  

blooded).

Aves  

are  

warm-blooded  

animals  

with  

feathers  

on  

their

bodies  

and  

forelimbs  

modified  

into  

wings  

for  

flying.  

Hind  

limbs  

are

adapted   

for   

walking,   

swimming,   

perching   

or   

clasping.   

The   

unique

features  

of  

mammals  

are  

the  

presence  

of  

mammary  

glands  

and  

hairs

on the skin. They commonly exhibit viviparity.