Ce animals of this phylum are found in fresh or marine water. {Very few are 

terrestrial. This phylum includes Slugs, Oysters, Clams and Squids. Slugs are terrestrial, while others are water-living. . 

in this phylum about 50,000 species of living and 35,000 of fossil mollusca animals 

are included. The mollusca are the smallest animais like sand grain and also the largest invertebrate Giant Squids having 18 meters long size and 1800 kg weight. ) 

Characters: ti" They have soft, unsegmented body with bilateral symmetry with organ system 

2. 

<i 

10. 

organization. They are triploblastic i.e. body consists of three layers, ectoderm, mesoderm and 

endoderm. Alimentary canal is in the form of a straight or coiled tube with mouth and anus at the 

opposite side. Coelom is reduced, consists of few fluid filled cavities and make a part of 

hydrostatic skeleton. 

Appendages are not produced from their body. From the lower side of their body a special structure foot is developed, which helps 

in locomotion, sensation and feeding. The upper layer of the body is called mantle. It produces shell for the protection of 

the body. The head is distinct. Mouth, eyes and tentacles are present on the head. 

They have a dorsal visceral mass that includes major organs. In some animals a special structure is produced in their mouth, called radula. It is 

used for scraping, tearing and pulling of food. Respiration takes place by gills or lungs. In aquatic animals gills and in terrestrial 

animals poorly-developed lungs are present. 

Excretion is by kidneys. Circulatory system is of open type, but the cephalopoda have closed type of 

Circulatory system. Heart is present. Nervous system consists of a cerebral-ganglion, a pair of pedal nerves upto the 

foot and a pair of visceral body. 

or in 15. Reproductive organs are found either in the same body Fertilization is ig water. 

©) After fertilization the egg develops into a larva whi larva is of trochophore type