Characteristic features of different classes mentioned are as follows

(a) Chondrichthyes

(i) These are marine animals with a stream lined body.

(ii) Body is has cartilaginous exoskeleton and the skin is tough due to the presence of minute placoid scales.

(iii) These animals are predaceous.

(iv) Due to the absence of air bladder they have to swim constantly to avoid sinking.

(v) They are cold blooded animals e.g., Scoliodon (dog fish), Pristis (saw fish).

(vi) Some of them possess electric organs and poison sting (Torpedo and Trygon).

Osteichthyes

(i) These animals are found in both habitats, i.e., marine and freshwater. They bear bony endoskeleton.

(ii) Body is streamlined, posseses four pairs of gills covered by an operculum, on each side.

(iii) Skin is covered by cycloid or ctenoid scales.

(iv) Presence of air bladder is an additional feature providing buoyancy to animal.

(v) Heart is two chambered.

(vi) Sexes are seperate, fertilisation usually external, oviparous animal with direct development.

e.g., marine- Exocoetus (flying fish), Hippocampus (sea horse); freshwaterLabeo (rohu).

(b) Urochordata

(i) In urochordates, notochord is only present in larval tail.

(ii) They contain a tough outer covering called tunic.

(iii) Persence of pharyngeal slits during some stage of the life cycle and a muscular, post anal tail is the characteristic features in urochordates.

e.g., Salpa, Ascidia, Doliolum.

Cephalochordata

(i) Cephalochordates are exclusively marine animals.

(ii) The notochord in cephalochordates extends, from the heart to tail region and is persistent throughout the life.

(iii) Body-wall shows one cell thick, non-ciliated epidermis, connective tissue, striated muscle and parietal peritonium.

(iv) Fertilisation is external

e.g., Branchiostoma (Amphioxus or Lancelet).

(a) Similarities between aves and mammals

(i) The members of both the groups are homeotherms, i.e., warm blooded. They are able to maintain constant body temperature.

(ii) Heart is completely four chambered.

(b) Similarities between frog and crocodile

(i) The members of both the groups are poikilotherms, i.e., they lack the capacity to regulate their constant body temperature. They are cold blooded animals.

(ii) Frog and crocodile are oviparous animals.

(c) Similarities between turtle and Pila

(i) In both animals, body is covered with dry and cornified skin. In turtle, the epidermal covering is known as scales whereas in case of Pila, it is known as calcareous shell.

(ii) Both animals are oviparous.

(a) Ichthyophis does not possess limbs.

(b) Scoliodon (dog fish) is a cold blooded animal.

(c) Columba (pigeon) is a warm blooded animal.

(d) Naja naja (snake) possesses dry and cornified skin.

(e) Sycon (sponge) possesses canal system and bear spicules.

(f) Obelia bears cnidoblast.

(a) A viviparous animal is the one in which development of young ones is direct, later they provides nutrition to their offspring. It includes all mammals, except Platypus.

(b) A fish possessing a poison sting is Trygon (sting-ray). It is found on its tail. The poison released is strong enough to stun an animal.

(c) A fish possessing an electric organ Torpedo (electric ray). Electric organ in Torpedo is located in its big round pectoral fin which is just behind the eye. Large Torpedo can generate a fish killing current of up to 200 volts.

(d) An organ, which regulates buoyancy is air bladder present in class-Osteichthyes. (e.g., Exocoetus, Catla).

(e) Animal exhibiting alteration of generation is Aurelia or jelly fish of phylum-Cnidaria. It has two basic body forms called polyp and medusa are present. Polyp is sessile and cylindrical whereas medusa is umbrella shaped and free swimming form. Polyp produces medusa asexually and medusa forms polyps sexually (e.g., Obelia).

(f) Oviparous animal with mammary gland is Ornithorhynchus (duck-billed platypes). Duck bill platypus is the only mammal which lays egg and also bear mammary gland, which is a characteristic feature of mammals.