Phylum Mollusca forms the second largest phylum of the animal kingdom.
1. Molluscs are generally aquatic forms.
2. All molluscs are soft bodied and unsegmented animals.
3. They possess an organ system level of bod organizations.
4. They are bilaterally symmetrical, except in univalves
5. They are triploblastic animals
6. They are true coelomates but the coelom is reduced in adults
7. The body cavity is a haemocoel.
8. The soft body is usually protected by an unsegmented external shell. The shell is made up of calcium
9. Inside the shell the body is divided into 3 parts- head, Visceral hump and muscular foot.
10. The head regionshas sensory tentacles.
11. The mouth contains a file like rasping organ called radula for feeding.
12. The visceral hump contains the important internal organs. The visceral hump is covered by a soft and
spongy layer of skin called mantle or pallium.The mantle encloses a cavity called mantle cavity in which
feather like gills are present. The mantle secretes the shell.
13. The foot is soft and muscular. It is the locomotory organ
14. Many feather like gills are seen outside the body and below the mantle. Gills are the respiratory organs in
aquatic forms and pulmonary sacs in terrestrial forms.
15. Circulatory system is open type. Heart is chambered and valvular.
16. Digestive system is well developed, with salivary gland and liver
17. Excretory organs are kidneys
18. Nervous system consists of paired ganglia connected by nerves. Some sense organs are tentacles and
19. They are dioecious.Fertilization is external or internal
20. They are oviparous with indirect development.