Segmentation in the body is first observed in which of the following.

(i) The basic fundamental features such as level of organisation, symmetry, cell organisation, coelom, segmentation, notochord, etc., have enabled us to broadly classify the animal kingdom.

(ii) Four types of symmetry found in animals are: spherical symmetry, radial symmetry, biradial symmetry and bilateral symmetry.

(iii) Based on the number of germ layers there are two organisations in animals namely diploblastic and triploblastic.

(iv) Metamerism or segmentation is the serial repetition of similar parts along the length of an animal.

(v) A body cavity is a fluid-filled space between the gut and the outer body wall of an animal.

(vi) Animals without coelom or body cavity are called acoelomates. e.g., Cnidaria and Platyhelminthes.

(vii) A pseudocoelom found in Nemathelminthes is not lined by mesoderm.

(viii) In protostomes the mouth forms from the first opening of the embryo namely blastopore, e.g., Annelida, Arthropoda and Mollusca.

(ix) In deuterostomes, the mouth never develops from the blastopore, although the blastopore may give rise to the anus. e.g., Echinodermata and Chordata.

(x) The animals are categorised into two major groups, non-chordates and chordates on the basis of the presence or absence of notochord at some stage of their life.

2. Non-Chordates:

(i) The members of the phylum 'Porifera', commonly known as sponges are the most primitive and simplest multicellular animals with characteristic flagellated choanocytes.

(ii) The phylum 'Cnidaria' (formerly called Coelenterata) comprises aquatic, sessile or free-floating animals with tentacles bearing cnidoblasts or nematocysts.

(iii) The ctenophores, formerly included in coelenterates, are marine animals with comb plates.

(iv) Phylum Platyhelminthes comprises flatworms which are mostly parasitic, some are free-living, mainly aquatic, marine or freshwater.

(v) Aschelminthes or Nemathelminthes comprises pseudocoelomate roundworms which include parasitic as well as non-parasitic roundworms.

(vi) Arthropoda is the largest phylum in the animal kingdom.

(vii) The phylum Mollusca consists of the soft-bodied animals, and it is the second largest animal phylum in number of species.

(viii) Echinodermata comprises spiny-skinned animals which are exclusively marine with a unique water-vascular system or ambulacral system derived from enterocoel.

(ix) The phylum Hemichordata includes the marine acorn worms and pterobranchs which have a cylindrical body with proboscis, collar and trunk.

3. Chordates:

(i) The phylum Chordata is named after the notochord, a stiff, and flexible rod-like structure along the back lying ventral to nerve cord.

(ii) The distinctive features of chordates are notochord, dorsal hollow nerve cord, paired pharyngeal pouches or clefts and a post-anal tail.

(iii) Urochordates or tunicates have notochord and nerve cord in free-swimming larva only.

(iv) In Cephalochordata, notochord and nerve cord are found along the entire length of the body and persist throughout life.

(v) Few vertebrates do not possess jaws (Agnatha) whereas most of them possess jaws (Gnathostomata).

(vi) Agnatha is represented by the class Cyclostomata.

(vii) Classes Chondrichthyes and Osteichthyes bear fins for locomotion and are grouped under Pisces.

(viii) The Chondrichthyes are fish with cartilaginous endoskeleton and are marine.

(ix) The Osteichthyes are fish with bony endoskeleton and are marine or freshwater.

(x) Most amphibians are biphasic, having an aquatic stage, as well as a terrestrial stage.

(xi) Reptilia is the first class of terrestrial vertebrates characterised by the presence of dry and cornified skin.

(xii) Reptiles are predominantly terrestrial, carnivorous, burrowing or creeping, cold-blooded (ectothermic) or poikilothermic, tetrapod vertebrates.

(xiii) Aves (birds) are feathered bipeds, air-breathing, truly flying vertebrates (other being bats from Class Mammalia), the first vertebrates to have warm blood. In birds, forelimbs are modified as wings for flying.

(xiv) Mammals are hair-clad, mostly terrestrial, air-breathing, warm-blooded (endothermal) vertebrates. Most important character of mammals is the presence of mammary glands.

(xv) Mammals are viviparous, except egg-laying monotremes, developed uterine and foetus nourishes through placenta.