Biology Sikshya

  L., mollis=soft bodied

• They are mostly marine, some are fresh water and a few are terrestrial in habitat.

• They may be herbivorous, predators, free-living or nocturnal.

• Triploblastic animals i.e., body derived from 3 germ layer (ectoderm, mesoderm and endoderm).

• Body is divisible into head, mantle, foot and visceral mass.

• Body is unsegmented (except in class Monoplacophora).

• Organ-system grade of body organization.

• Coelom is deadly reduced and is confined to the pericardial, renal, gonodial, and mantle cavity.

• A ventral muscular foot is present for the locomotion of the animal.

• Shell if present serves as exoskeleton.

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• Respiration takes place by general body surface, ctenidia or by pulmonary sac or lungs.

• Digestive system is complete type. It is provided with a digestive gland called Hepatopancrease. Anus opens into pallial cavity.

• Buccal cavity contains a rasping teeth like structure called radulla.

• Excretion occurs by a paired metanephridia.

• Circulatory system is open type. A heart is present dorsally. Respiratory pigment is haemocyanin (contains copper).

• Nervous system consists of a paired cerbral ganglion, nerves and connectives.

• Sense organ includes eyes, statocyst and an olfactory organ called Osphradium.

• Reproduction is sexual. Development may be direct or indirect.

• Fertilization may be external or internal.

• Mostly dioecious and a few are monoecious.

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