Phylum Mollusca, Biology tutorial

Characteristic of Phylum Mollusca:

i) They are bilaterally symmetrical

ii) Usually, there is distinct head and muscular foot; dorsal body wall forms mantle folds that surround mantle only.

iii) Employs gills and lungs for respiration,

iv) The hard, calcareous shell secreted by mantle, protecting body is common.

v) They contain soft unsegmented bodies comprising of head, viscera mass and foot.

vi) Free living aquatic forms (freshwater or marine), few are amphibious.

vii) Body is soft and unsegmented surrounded in glandular mantle covered by generally shell.

viii) Coelom is decreased to pericardial cavity around heart.

ix) Digestive system is complete.

x) Respiratory organs are in form of gills known as ctenidia.

xi) Locomotor structure is represented by muscular foot.

Categorization of Phylum Mollusca:

There are 3 significant classes of phylum. These comprise:

i) Class pelecypoda or Lamelli branchiata or Bivalea. For e.g. Oysters, Clams, and Mussels etc.

ii) Class Cephalopoda. For e.g. Octopus, Nautilus, Cuttefish, squids, etc.

iii) Cypraca (cowrie) patella (unpet) etc.


Snails are member of mollusk. It moves by gliding along on the muscular foot. This muscle continually secretes mucus, that uses their movement and later dries to form silvery slime trail which signals presence of either pest. Snails belong to class gastropods that are largest class of Mollusca. Snails are most energetic at night as well as on cloudy or foggy days. On sunny days they look for hiding places out of heat and bright light. Frequently only clues to the presence are silvery trails and plant damage. Areas with mild winters, like southern coastal places, snails can be energetic throughout year. They vary from other groups in 4 main ways based on external features. Features are given below:

i) There is growth of distinct head

ii) Body exhibits dorsoventral elongation.

iii) The spiral asymmetrical shell which serves as protective retreat for animal

iv) Visceral mass experiences a 90 to 180 degree twist with respect to head and foot, phenomenon called as torsion.

v) The garden snail is the herbivore.

vi) Mouth contains a ribbon-like rasping tongue known as radula.

vii) The radula is enclosed with horny curved teeth.  Radula is rubbed rear and forwards against leaf and tears off small pieces of vegetation in the related way to action of the rasp.

viii) Diaphragm is utilized to fill lung with air and assists force oxygen in blood

ix) O2 is carried away from lungs in blood.  In molluscs O2 is hold by haemocyanin (similar to haemoglobin in mammals).

x) Opening to lung can be open and closed by the muscular valve - closing of opening helps decrease water loss.

Adaptation of Gastropoda to their Environment:

i) Their foot is flat creeping sole. Ciliated and glad cells situated in foot secrete mucus over which animal moves (Locomotion).

ii) In burrowing forms, foot acts like the plough and anchor Limpets, slipper snails are adapted for clinging to rocks and shells.

iii) In the group of pelagic gastropods (sea-butter fish), foot is altered in effective fun-like swimming organs

iv) Land snails between gastropod have their mantle highly vascularized and converted limbs lung for objectives of gas exchange.

v) Gastropods show the diversity of feeding habits. There are carnivores, herbivores, scavengers, parasites.

vii) Fertilization is generally internal and fertilized eggs may at times be surrounded in egg capsules

viii) Snail moves by ripples of muscle reduction moving down its foot.  There is gland behind mouth that releases mucus to decrease friction and prevent damage to foot.

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