(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.

(i) The word Arthropod means 'jointed legs'.
(ii) The body is divided into head, thorax and abdomen.
(iii) The coelomic cavity is filled with haemolymph (blood).
(iv) Open circulatory system is seen.
(v) Exoskeleton has to be shed periodically in a process called moulting.
(vi) Small arthropods directly absorb oxygen through their body surface.

(i) Coelom refers to a fluid-filled cavity inside the body.
(ii) It separates the digestive tract and other organs from the body wall.
(iii) A true body cavity or coelom is one that is located within the mesoderm.
(iv) Organisms like the earthworm are called coelomates or eucoelomates because they have true coeloms.
(v) Tapeworm is an example of an acoelomate because it does not have a body cavity.

Absence of Cranium.

Water vascular system and tube feet.

Phylum Mollusca.

Jointed legs.

Greek.

Embryonated eggs.

Frogs are live in aquatic and land. So this double life is expressed as amphibians. So frogs are called as amphibians.

1. Hemichordata
2. Cephalochordata
3. Urochordata

The respiratory organs of amphibians are gills, skin, bucco-pharynx and lungs.

Earthworm is called as "Friends of farmers", because when they move, they tilt the soil to get free entry of air.