Biology XI Content
- They have soft, unsegmented and muscular body.
- They are aquatic (usually marine, some are fresh water) and some are terrestrial found in damp and moist places.
- They are triploblastic i.e. body contains 3 germs layers- ectoderm, mesoderm and endoderm.
- They are bilaterally symmetrical except gastropods (E.g. Helin). Gastropods are asymmetrical due to torsion (i.e. coiling of internal organs).
- They are coelomate and body contains haemocoelom.
- They possess organ-system grade of body organization.
- Body is covered by calcareous shell and divided into head, mantle, foot and visceral mass.
- They exhibit holozoic form of nutrition.
- They have a complete and straight alimentary canal. Digestion is extra-cellular.
- Respiration occurs by gills or ctendia (inaquatic forms) and lungs in terrestrial forms.
- Excretion occurs by metarephridilla or kidney.
- Circulation is of opened type i.e. blood flows through open spaces inside body not in blood vessels.
- Locomotion takes place by the muscular foot`s contraction and relaxation.
- Sensory organs are eye, tentacles, osphradium, etc
- Development is indirect i.e. larval form is known as trocophore larva or Veligar larva.
On the basis of nature and type of muscular foot, they are classified as:-
- They are marine forms.
- Head bars tentacles.
- Body is symmetrical with dome shaped mantle.
- Foot s disc like with a flat, creeping sole.
- The shell is flattened limpet-shaped with spirally coiled protoconch.
- They form dorso-ventrally flattened body.
- A small head is present. It is not distinct from the rest of the body.
- Eyes and tentacles are absent.
- Shell may be absent or consists of a row of plates.
- Foot may be large, reduced or absent.
- Scaphodia (boat-like &pointed and used for burrowing feet)
- They are exclusively marine.
- Body is elongated and enclosed in the tusk like shell.
- The shell is tubular and opens at both ends.
- Head is rudimentary. Eyes are absent.
- Foot is elongated and conical. It is used for digging.
- Gastropoda (ventral muscular foot rising from a stomach)
- They are terrestrial, fresh water as well as marine.
- Body is covered by spirally coiled univalve (single valve shell)
- Head is distinct with tentacles.
- Foot is large, muscular and flat.
- Body is asymmetrical in adult due to torsion.
Example Pila, Helia, Limaxetc
- They are mostly marine and few are fresh water.
- Body is laterally compressed and bilaterally symmetrical.
- Body is covered with bivalve shell. Each shell consists of two valves morably hinged dorsally.
- Head is absent. Eyes, tentacles and radula are absent.
- Foot is wedge or hatchet shaped for digging.
- Foot arises from head, which is modified into ariss with suckers.