On the basis of position of notochord, the different prochordates are classified as follows.
1. Sub-phylum Hemichordata : The notochord is persistant as the stomochord in the anterior region of the animal. e.g., Balanoglossus.
2. Sub-phylum - Cephalochordata : The persistent notochord extends forward beyond the brain. e.g., Amphioxus.
3. Sub-phylum - Urochordata : The notochord is confined to the tail region of the larva, e.g., Asicidian.

Reproductive characters of mammals differ from those of Aves by
1. The presence of mammary glands.
2. Testes lie outside the body cavity enclosed in scrotal sacs.
3. Alecithal eggs.
4. Fertilization is always internal.
5. They give birth to young ones and feed their youngones with milk.
6. Placenta is a characteristic feature of mammals.

This animals are characterized by the possession of a body cavity called coelom. Some animals of annelids have movable bristles called setae it is used to locomotion. Nephridia is the excretory structures and help to remove metabolic waste. They have central nervous system with a brain.

(i) Birds (Aves) are the first homeothermic vertebrates with spindle shaped body.
(ii) They have two pairs of limbs, in that forelimbs are modified as wings.
(iii) The feet are covered with scales, while the body is covered with feathers.
(iv) A horny beak is present.
(v) Respiration is through spongy lungs.
(vi) The bones are filled with air, so they are called pneumatic bones.

The bones filled with air.

Water vascular system and tube feet.

Jointed legs.

Greek.