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The sub-phylum vertebrata is divided into five classes;
(a) Fishes, amphibians, reptiles, birds and mammals.
Fishes are aquatic vertebrates with fins as locomotor organs and gills as respiratory ones; cold-blooded, i.e., body temperature varies with the ambient. Body is streamlined; ten pairs of cranial nerves present; skin covered with scales; heart is two chambered and contains only deoxygenated blood; red blood cells are nucleated.
There are two subclasses
(a) Cartilaginous fishes as sharks, sting ray, and saw fish; and (b) bony fishes as seahorse.
(b) Amphibians lead a double life, living equally well on land and water; cold blooded with two pairs of limbs (the caecilians Are without limbs and the sirens have only the hind limbs. They are mostly oviparous and their development often involves metamorphoses, involving an intermediate larval form. Examples: frogs, toads, salamander and newts.
(c) Reptiles are cold blooded air breathing vertebrates which have adapted themselves completely to land. Some, however, have secondarily taken to water; normally two pair of short limbs with claws, but in some, they are reduced, modified or lost (as paddles in turtles, lost in snakes). There are 12 pairs of cranial nerves; most are oviparous, snakes are viviparous, no metamorphosis.
Examples : turtles and tortoises, (former are aquatic); lizards (garden, wall, flying); chameleons ; snakes-cobra, krait, and vipers are the common poisonous snakes; rat snake and pythons are non-poisonous snakes; crocodiles and alligators.
(d) Birds are warm-blooded, adapted to aerial life; streamlined body covered with feathers; fore limbs are modified as wings for flight, hind limbs are used for perching, walking or swimming; some are incapable of flight (running birds-ostrich and kiwi. Flying birds are-pigeons, ducks (aquatic), parrots, kites, owls (nocturnal ).
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