All adult multicellular organisms typically possess a concentric arrangement of tissues in the body. These tissues are derived from the three embryonic cell layers called germinal layers.

(i) The outer layer is the ectoderm, the middle layer is the mesoderm and the innermost layer is the endoderm.

(ii) Endoderm layer is associated with the formation of the stomach, colon, liver, pancreas, urinary bladder and other vital organs in an organism.

(iii) Mesoderm forms the main structural components of the body like the skeletal muscles, the skeleton, the dermis of the skin connective tissue, etc.

(iv) Ectoderm is associated with the formation of CNS, eye lens, ganglia, nerves and glands.

(v) The body cavity that is lined by mesoderm is called coelom, and the animals possessing coelom are called as coelomates. e.g., phylum-Annelida, Mollusca, Arthropoda, Echinodermata, Hemichordata and Chordata.

(vi) In some organisms, body cavity is not lined by mesoderm, instead mesoderm is present in the form of scattered pouches in between ectoderm and endoderm, Such body cavity is called pseudocoelom and animals possessing there stusturs are refered to as pseudocoelomates e.g., Ascaris.

(vii) The animals in which there is complete absence of body cavity are called acoelomates. e.g., Platyhelminthes.

Amphibians

(i) They possess dual life as they can live in aquatic as well as terrestrial habitat. They are ectothermic (cold blooded).

(ii) They are tetrapods (4 limbs) which facilitate movement land.

(iiii) Their limbs are evolved from the pectoral and pelvic fins.

(iv) Skin is thin, covered by mucus and mostly remain moist, also serves as an accessory source oxygen.

(v) They are both gill and lung breathers; usually gills appear in th larval stage, replaced by lungs in the adults stage.

(vi) They possess three chambered heart with two atria and one ventricle.

(vii) The fertilisation is mostly external, females are oviparous.

(viii) Larva is a tadpole, which metamorphosis into adult thus shaving metamorphosis. e.g., Rana frog, Nectureus (mud puppy), Salamandera (salamander).

Reptiles

(i) In reptiles, creeping and crawling mode of locomotion is found.

(ii) They are mostly terrestrial animals and their body is covered by dry, and cornified skin, epidermal scales or scutes.

(iii) Lungs are well developed and present in all stages of life.

(iv) Toes possess claws.

(v) Appendages are well adapted for the movement on land.

(vi) Heart possesses a partially divided ventricle and 2 atria.

(vii) They lay amniotic eggs which are incubate on land.

(viii) They are poikilothermic or cold blooded animals. Temperature is regulated mechanically and not metabolically by moving in and out; heat source is usually the sun.

(ix) fertilisation is internal They are oviparous and development of young ones is direc.

e.g., Chelone (turtle), Naja (cobra), Crocodicus (crocodile).

Mammals are most adapted among the vertebrates and the following features explain the justify this statement.

(i) They are found in variety of habitats, i.e., polar ice caps,deserts, mountains, forests, grasslands and dark caves.

(ii) They have better developed brain. The brain bears large cerebellum and cerebrum.

(iii) The most unique mammalian characteristic is the presence of milk producing glands (mammary glands) by which mammals nourish their young ones.

(iv) Mammals are homeotherms (warm blooded), i.e., they are capable of maintaining their body temperature regardless of to the surrounding environment.

(v) They possess oil secreting glands (sebaceous glands) and sweat glands (sudoriferous glands) in the skin.

(vi) They possess two pair of limbs, that are well adapted for walking, running, climbing, burrowing, swimming and flying.

(vii) fertilisation is internal. They show viviparity with few exceptions e.g., oviparous Ornithorhynchus (platypus) and the development is direct. e.g., viviparous - Macropus (kangaroo), Rattus (rat), Canis (dog), Delphinus (common dolphin), Panthera tigris (tiger), etc.