The science of classifying organisms is called taxonomy. To date there are five kingdoms, of which one is the Animalia, which is made up of animals. To classify this group we divide them into two the Non chordates and the chordates.
The chordates are characterized by the presence of notochord at some stage of development, the pharyngeal gill-slits present at some stage of life and their single, dorsal, tubular and hollow nerve chord, not to mention their heart in the ventral position. The animal kingdom are classified into the following groups:
Sub- Phylum – Urochordata or Tunicata which are marine animals whose body is surrounded by a leathery covering (tunic). Larvae are free swimming, notochord is only in the tail in larvae and after settling on seashore, they get transformed into sessile adults. They are generally hermaphrodite. Examples include Ascidians, Doliolum, Oikopleura.
Sub-phylum – Cephalochordata consists of small fish-like marine animals whose notochord extends along the entire body and pharynx is large with numerous gill- slits. Sexes are separate. Example is Amphioxus or lancelet.
Sub-phylum – Vertebrata, in this the notochord is replaced by a vertebral column and the head is well developed. Brain is protected in a cranium and the endoskeleton may be cartilaginous or bony. They may be jaw-less (Agnatha), with jaws (Ganathostomata).
Class Cyclostomata: They have sucking mouths without jaws, skin is soft, devoid of scales and characterized by the absence of paired appendages. They have a cartilaginous endoskeleton and are mostly ectoparasites. Examples are petromyzon, Myxine.
In Gnathostomata, the super classes are two.
Class Pisces (Fishes) are cold blooded, aquatic and have stream-lined body and fins for swimming. Tail fin exists for direction changing and exoskeleton is the form of scales. Endoskeleton may be cartilaginous or bony while respiration occurs through gills. They also have eyes without eye-lids. Examples are dogfish and Rohu.
Class Amphibia comprises of organisms which are cold-blooded belonging to the freshwaters or terrestrial. Characterized by limbs which are two pairs, digits without claws, absence of exoskeleton, smooth, moist skin for respiration, presence of ear drum and eyes which are protruding provided with eyelids. Examples are frog, toad and salamander.
Class Reptilia comprises of organisms which are cold-blooded, crawling on the ground, with their dry skin bearing scales or bony plates. They are characterized by the presence of a neck and the absence of external ear. Digits are provided with claws. Examples are Tortoise, Wall lizard, Snake.
Class Aves (Birds) are warm-blooded animals having stream lined body for lower air resistance during flight and their forelimbs are modified into wings. They are characterized by digits which are clawed and covered with scales, exoskeleton in the form of feathers and the presence of a neck and beak. Examples are Parrot, Pigeon and Duck.
Class Mammalia are distinguished by the presence of mammary glands, a body that is divided into Head-Neck-Trunk-Tail. Digits are provided with nails, claws, hooves, the exoskeleton is in the form of hairs or fur and the external ear is absent. They are warm-blooded animals. Examples are Bat, Squirrel, Rat, Lion, Monkey, Man.
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