Introduction to Phylum Mollusca:
Phylum Molluscs is one of the largest phyla among invertebrates and comprises more than 50,000 living species and approx 35,000 fossil species. They include snails, clams and squids etc. Rich fossil record of phylum is because of presence of mineralized shell in several species. Though most of the molluscs are aquatic, occupying both fresh and marine waters, some species live on land also.
General Features of Phylum Mollusca are:
1. Bilaterally symmetrical.
2. Usually there is distinct head and muscular foot; dorsal body wall forms mantle folds that enclose mantle cavity.
3. Frequently there are gills and lungs for respiration, created by modified mantle.
4. Hard, calcareous shell secreted by mantle, protecting soft body, is common.
5. Coelom is restricted to spaces around heart (pericardial cavity), in gonads and in kidneys.
6. Circulatory system is open kind in most forms, with heart, blood vessels and sinuses.
7. Excretory organs are metanephridia, sac-like kidneys, opening proximally in pericardium and distally in mantle cavity.
8. Nervous system comprises of well-developed ganglia (cerebral, pedal, pleural and visceral) majority of them concentrated in ring with connectives and commissures.
Molluscs usually measure many centimeters in length. Although mollusks seem to be heterogeneous assemblage of animals, they contain basic body plan. Visceral mass is enclosed by epidermis, the mantle or pallium. Mantle secretes overlying protective shell and edges of mantle are most active in secreting shell.
The shell contains 3 layers:
i) The outer, homy layer composed of conchiolin, a modified protein. This layer is known as periostracum. It is protective to underlying layers and is secreted by fold of mantle edge only.
ii) Middle prismatic layer is composed of prisms of calcium carbonate dorsally packed, in the matrix of protein. This layer is also secreted by glandular margin of mantle.
iii) Innermost is nacreous layer. This is calcareous material, and is layered down by mantle surface continuously. This is iridescent mother-of-pearl of several molluscs. Pearl is created between mantle and shell in response to trapped foreign particles when they get enclosed with nacreous layer.
Classification of Mollusca:
Phylum Mollusca comprises seven classes. They are:
i) Class Monoplacophora:
Bilaterally symmetrical, with broad flat foot and single shell, mantle cavity contains 5 to 6 pairs of gills; 6 pairs of nephridia of which 2 function as gonoducts; radula present; sexes separate. Extant species are comprised in three genera. The monoplacophorans can be considered as ancestors of all molluscs.
The salient characters of Monoplacophorans:
ii) Class Polyplacophora:
Bilaterally symmetrical lengthened, dorsoventrally flattened; head reduced, with radula, shell comprises of 8 dorsal plates, foot flat and broad; numerous gills along sides of body; sexes separate. Trochophore larva in life cycle; no veliger larva. It includes chitons. They live joined to rocks. Body is dorsoventrally flattened and is enclosed by eight overlapping shell plates: therefore the name Polyplacophora.
Significant general characters are as follows:
iii) Class Aplacophora:
It is Worm-like, does not have shell, head or excretory organs; mantle with chitinous cuticle or scales or spicules; mantle cavity posterior. Aplacophorans are the group of small worm like molluscs lack of any shell, known as solenogasters. They occupy ocean depths, though few shallow water species are also known.
Features of Class Aplacophora are:
They are less than 5 cm in length with poorly developed head and cuticle covered integument in which calcareous scales or spicules are embedded. Upward rolling of mantle margins give worm like appearance to creeping species. Burrowing forms feed on small organisms and deposited materials, and creeping species feed on cnidarians. Radula may or may not be there. Most forms are hermaphrodites and gonoducts open in mantle cavity. Eggs may develop directly in adult or pass through trochophore larval stage.
iv) Class Gastropoda:
Body asymmetrical, illustrates torsion or its effects; shell coiled in most, well developed head with radula; large flat foot; gills one or two or with pulmonary cavity (lung); generally with single auricle and single kidney. Nervous system having cerebral, pedal, pleural and visceral ganglia; generally with trochophore and veliger larvae.
Gastropods, the largest class of molluscs, illustrate extensive adaptive radiation, adapted to life for bottom and spelagic and littoral existence in sea. They have also invaded fresh water and several have successfully conquered land also. Gastropods vary from other groups in given ways.
v) Class Bivalvia:
Body within bilobed mantle surrounded in two-valved shell; head reduced; mouth with labial palps but no radula; foot wedge shaped; plate-like gills; sexes separate; trochophore and veliger larvae.
Class Bivalvia is also called as Pelecypoda or Lamellibranchia. It comprises clams, oysters and muscles. Body of these molluscs is laterally compressed and shell is created of two valves hinged dorsally. Shell covers body completely. Foot of animal is also laterally compressed. Bivalves have most spacious mantle cavity among molluscs. Large gills which are surrounded in mantle cavity function as organs of gas exchange and food gathering apparatus producing ciliary currents helping in filter feeding.
vi) Class Scaphopoda:
Body within the single tusk-shaped shell open at either ends; foot conical; mouth has radula and tentacles; does not have head; gills; sexes separate; trochophore larva.
Class Scaphopoda comprises marine burrowing forms commonly called as tusk shells or tooth shells, because of resemblance of shell to tusk of the elephant. Size of shells generally ranges on average from 3 to 6 cm. Shells are generally white or yellowish.
vii) Class Cephalopoda:
Shell reduced or absent except in Nautilus; head well grown with well developed eyes; head bears tentacles; foot altered in siphon; well developed, brain with fused ganglia; sexes separate; development direct. Cephalopods are most active and extremely organized of all molluscs, adapted for pelagic, free swimming life. Group comprises cuttlefishes, squids, octopods and Nautilus. Body is prolonged in dorsoventral axis and this has in effect become antero-posterior axis. Mantle cavity is now ventral. Head projects in circle of large prehensile tentacles or arms at anterior region. Tentacles are homologous to anterior portion of foot.
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