Mollusca

Molluscs → soft

LabmarckMolluscshe term molluscs. It is the second largest phylum after Arthropoda. The study of molluscs is called conchology. Kokichi Mikimoto of Japan is known to be the father of pearl industry.

General characteristic features of phylum Mollusca:

• They are mostly freshwater or marine and few are terrestrial living in a damp place. They are distributed worldwide.
• The body is soft, unsegmented and without appendages.
• They are triploblastic.
• They are bilaterally symmetrical except gastropods which are asymmetrical due to coiling.
• They are haemo coelomate animals.
• The body is visible in four parts: head, mantle, foot and visceral mass. The body is covered by a calcareous shell. In some, it may be internal or absent.
• Single ciliated epidermis.
• Locomotion takes place by ventral muscular foot. It may be modified for creeping or burrowing type.
• The alimentary canal is simple and straight.
• The buccal cavity bears radula for feeding.
• Respiration takes place by gills or ctenidia and lungs in terrestrial forms.
• It is an open type with many large blood spaces in the tissues. Blood is green due to the presence of a copper containing a pigment called haemocyanin.
• Excretion takes place by paired metanephridia or kidney.
• The nervous system is consists of a brain, many ganglia connected by connectives or commissures.
• Sense organs are eyes, tentacles, osphradium, statocysts etc.
• Sexes are usually separated, but some are hermaphrodite.
• Fertilisation may be external or internal. Most of them are oviparous and few are vaporous.
• Development is indirect having a larval stage-trochophore larva and veliger larva.

Classification:

There are 10-18 thousand living species. Mollusca is classified into five classes, mainly on the basis of nature and type of muscular foot.

Gastropoda:

• The foot is large and flat, used in creeping.
• Body covered by a hard shell.
• Asymmetrical.
• Marine, freshwater, terrestrial.
• Head with eyes and sensory tentacles.
• Example: Pilia, Slug, Helix etc.

Scaphopoda:

• The foot is pointed, used in burrowing.
• Body covered by a hard shell.
• Bilaterally symmetrical.
• Exclusively marine.
• Head, eyes and sensory tentacles are absent.
• Example: Dentalium, Cadulus.

Pelecypoda:

• The foot is often hatched shaped, laterally compressed, extending between mantle lobes used for burrowing.
• The body is covered by a shell of two valves.
• Bilaterally symmetrical.
• Marine or freshwater.
• Head, eyes and sensory tentacles are absent.
• Example: Unio, Mytilus, pearl oyster.

Cephalopoda:

• The foot is modified into arms with suckers.
• Bilaterally symmetrical.
• Marine or freshwater.
• Head with well-developed eyes and sensory tentacles.
• Example: Octopus, Sepia etc.