Phylum

Phylum : Arthropoda:-These are the most successful group of animals. They outnumber all other animals in population strength. The body is segmented. It is covered by a hard exoskeleton made of chitin . During growth the exoskeleton is shed (moulting of ecdysis).The legs or paired appendages are jointed. The head

region has a pair of prominent compound eyes. Each compound eye is made up of several photoreceptor sub units called Ommatidia.

They have an open circulatory system without vessels. The body cavity is filled with a fluid called haemolymph. Such body cavity is known as haemocoel. These are unisexual, exhibiting

sexual dimorphism. The young forms produced are invariably called the larvae. The larvae undergo metamorphosis and develop into adults. This Phylum comprises five Classes,

Class Onychophora: It includes small worm like Peripatus. Peripatus

shows Annelidan and Arthropoda characters. Hence this may be considered as a  connecting link between the two groups.

Class Crustacea :- The examples for this class are  prawns ,

Crabs and lobsters . The dorsal body surface is covered by a sheild like carapace .

Class Myriapoda:- It includes centepedes and  millipedes. These organ-isms have a distinct head and simple eyes. The centepedes have a pair ofpoison claws. The body consists of numerous segments, bearing pairs of legs.

Class Insecta:- It comprises the common insects. The body is divided into head thorax and abdomen.In several insects, the adults have two pairs of wings on the thorax. Respiration happens through the tracheal system.

Class Arachnida :- It includes scorpions, spiders, ticksand mites. The body is divided into  cephalothorax and abdomen. There are four pairs oflegs attached to the cephalothorax.

Class Insecta:- It comprises the common insects. The body is divided into head thorax and abdomen.In several insects, the adults have two pairs of wings on the thorax. Respiration happens through the tracheal system.

Class Arachnida:- It includes scorpions, spiders, ticks and mites. The body is divided into cephalothorax  and abdomen. There arefour pairs of legs attached to the cephalothorax.

Phylum Mollusca:- It is a very successful and diverse group of animals. Considered to be the second largest group of animals with regard to species number. These are soft bodied animals without segmentation. The body is divided into head, muscular foot and

visceral mass. The body is covered by a  mantle and a shel

l. Respiration happens through gills (ctinidia) in the mantle cavity. The most common larva is a trochophore larva. There are seven classes of which three are more prominent.

Class Pelecypoda or Bivalvia:- These are aquatic molluscs having bivalves.They burrow in mud and sand. The body is laterally compressed.(eg) mussels , clams, oysters.

Class Gastropoda:- These are either aquatic or terrestrial molluscs. They posses a spiral shell.The foot is large and flat. They have well developed head with tentacles and eyes. (eg) snails, slugs, and limpets.

Class Cephalopoda:- These are mostly marine. They are adapted for

swimming. The foot is modified into eight to ten long tentacles in the head region. The shell is either internal or absent. (eg) Octopus, Loligo, Sepia.

Phylum Echinodermata:- These are marine organisms. While the adults are radially symmetrical the larvae remain bilaterally symmetrical. The mouth is on the lower surface. They have a

water vascular system with tube feet. eg. star fishes, brittle stars,

sea urchins and  sea-cucumbers .