Invertebrates
The animals without a backbone are called invertebrates. There are 8 types of Invertebrates. They are :
Porifera : This group consists of animals that live in fresh and salty water. Until the late eighteen century , people thought that sponges were plants. That was because adult sponges do not move as most other animals do. They have a large opening at the top and have many small openings called pores, all over their body. Water along with food enters through these pores. eg sycon and spongilla.
Cnidaria : Animals in this group have a bag shaped body , with one open end called mouth and a large central body cavity. around the opening, a number of arm like structures called tentacles are present . This body is radially symmetrical. eg Hydra , jellyfish and sea anemone.
Platyhelminths : This group has organisms which have flat, leaf-like or ribbon-like soft body.They have a bilaterally symmetrical body. They lack a body cavity and havr ony one opening. Each organism possesses both male and female reproductive organs. They are called hermaphrodites. eg liver fluke , plan aria and tapeworm.
Nemathelminthes : This group has a long ,cylindrical ,unsegmented ,bilaterally symmetrical body . They have an opening called mouth at the anterior end of the body. Another opening called Anus is at the posterior end . They are most commonly found in the intestine of some hosts as parasites ,however , some inhabit freshwater , marine and terrestrial environment. eg pinworm,Ascaris and whipworm.
Annelida : Animals in this group have a soft segmented and bilaterally symmetrical body. They have a body cavity. They have special organs of excretion called nephridia. Most of the organisms are hermaphrodites, though some may have separate gender. eg earthworms , leeches etc.
Arthropoda : This is the largest group of invertebrates. Animals belonging to this group have a segmented body and jointed legs. Besides Jointed legs , all arthropods have a tough outer covering called an exoskeleton. The body is differentiated into head,thorax and abdomen . Their body is bilaterally symmetrical . eg prawn, crab,Spider,Ant,Millepede etc.
Mollusk : Animals in this group have soft body. In Fact , Mollusk means soft. Their body is often covered by a hard shell. Some mollusks have single shell. Others may have two shells that are attached on one side. Some of them like Octopus , have a shell inside their body . eg snail , cuttle fish , squid and oysters.
Echinodermata : They have radially symmetrical body with body part occurring in fives . the animals have sharp spines sticking out through their skin. Echinoderm means spiny skinned. They have a mouth on the lower side and an anus on the upper side. The animals belonging to this groups have a large number of tiny tube - like feet .
