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In this article we will discuss about the general characters and classification of Phylum Mollusca.
General Characteristics of Phylum Mollusca:
1. Habitat:
They are mostly marine. Many, however, occur in fresh water and some even in damp soil.
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2. Body Form:
The body of molluscs is un-segmented with a distinct head, muscular foot and visceral hump. Neopilina is a segmented mollusc.
3. Symmetry:
They usually show bilateral symmetry. In some molluscs like Pila, due to torsion (twisting) during growth, the adults become asymmetrical.
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4. Shell:
Shell is secreted by mantle. It is made up of calcium carbonate. Shell may be external (e.g., most of molluscs), internal (e.g., slug, cuttle fish, squid) or absent (e.g., Octopus).
5. Mantle (Pallium):
It is a thin, fleshy fold of dorsal body wall more or less covering the body. It encloses a space, which is called mantle cavity (= pallial cavity).
6. Body wall:
Single layered epidermis is usually ciliated. Muscles are un-striped and occur in bundles.
7. Body cavity:
Coelom is greatly reduced. It is restricted to pericardial cavity (space around the fieart), and to small spaces within kidneys and gonads (testes and ovaries). Spaces amongst the viscera (soft organs) contain blood and form haemocoel.
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8. Digestive tract:
It is complete.
9. Blood vascular system:
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It is open type. It includes dorsal heart, arteries that open into sinuses; (spaces) and veins. Blood is usually blue due to the presence of a copper- containing blue respiratory pigment called haemocyanin. Among the molluscs, cuttle fish are exceptional in having closed blood vascular system.
10. Respirator, organs:
These are gills (cteoidia). Mantle and pulmonary sae (in semi-terrestrial form).
11. Excretory organs:
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Excretory organs are one or two pairs of sac- like kidneys. Gills are also excretory in function. Ammonia is chief excretory matter.
12. Nervous System:
The nervous system comprises paired cerebral, pleural, pedal and visceral ganglia joined by the nerve connectives and conumssures. Connectives connect dissimilar ganglia; however, commissures connect similar ganglia
13. Sense Organs:
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In many molluscs, eyes are present over stalks called ommatophores. Statocysts (balancing organs) may be present. Osphradium is present in some molluscs for testing chemical and physical nature of water.
14. Sexes:
The sexes are generally separate but some are hermaphrodite.
15. Development:
They are oviparous. The development is either direct or indirect (metamorphosis). When the development is indirect it includes a characteristic larva, liger, trochophore or glochidium. Asexual reproduction is absent.
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Unique Features:
(i) Mantle covers the body,
(ii) Mantle may be surrounded by shell, and
(iii) Nervous system consists of cerebral, visceral, pleural and pedal ganglia.
Advancement over Annelids:
(i) Shell is present in many individuals,
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(ii) In some forms, a lungs is present for pulmonary respiration, and
(iii) Better developed sense organs such as eyes, statocysts and osphradia.
Classification of Phylum Mollusca:
Phylum Mollusca are divided into six classes.
Class 1. Monoplacophora (Gk. monas- one, plax- plate, pherein- bearing):
The shell is spoon or cup shaped. They have die characters of both the phylum Annelida and phylum Mollusca.
Example:
Neopilina.
Class 2. Amphineura (Gk. amphi- both + two neuron = nerve):
There is a present non-ganglionated nerve ring around mouth with two pairs of interconnected nerve cord.
Examples:
Chaetopleura (Chiton).
Class 3. Scaphopoda (Gk. scapha- boat, podos-foot):
Shell is tubular and open at both ends.
Example:
Dentalium.
Class 4. Gastropoda (Gk. gaster- belly, podos- foot):
Shell is made up of one piece.
The early embryo is symmetrical but during development the body twists showing torsion so that the body becomes asymmetrical. It includes the largest number of molluscs e.g., Pila, Umax, Cypraea (Cowrie), Helix (garden snail), Aplysia (sea hare), Doris (sea lemon), Limnaea, (pond snail), Planorbis, Patella (true limpet), Turbinella (Shankh), Creseis (Sea butterfly).
Class 5. Pelecypoda or Lamellibranchiate or Bivalvia (Gk. pelekus- hatchet Podos foot):
Shells is made up of two halves.
Examples:
Unio, Mytilus (Sea mussel), Teredo SpwoS EnZ (razor shell or razor clam), Solen (razor fish or razor shell), Ostrea (edible oyster), Pecten (scallop), Pinctada (Pearl oyster).
Class 6. Cephalopoda (Gk. kephale- head, podos- foot):
Head and foot region combined and modified into a structure which has eyes and eight tentacles, hence the name cephalopod or ‘head foot’. Cephalopods are regarded at the top of invertebrates evolution in terms of learned behaviour they exihibit. Shell is external (Nautilus), internal (sepia) or absent (octopus).
Examples:
Sepia, Loligo, octopus, nautilus, (pearly nautilus). Some cephalopods are the largest invertebrates.
Evolutionary Precursor of Molluscs:
A “living fossil” named Neopilina discovered in 1952 from the Pacific Ocean, shows metameric segmentation which is not a characteristic of molluscs. Neopilina has 8 pairs of muscles, 5 or 6 pairs of gills, and 5 pairs of nephridia.
Metameric segmentation and presence of the trochophore larva in both annelids and molluscs suggest that molluscs have descended from the annelids. Thus, the annelids are the evolutionary precursor of molluscs. Neopilina is a “connecting link” between Annelida and Mollusca.
Chiton-The coat of mail shell:
Chiton is marine and found attached to rocks by its foot the shell consists of a row of eight plates.
Dentalium— The Elephant’s tusk-shell:
It is a marine mollusc. Dentalium is found burrowing in sand. The shell is slightly curved, tubular and opens at both the ends. There are present filamentous tentacles called the captacula, which are useful in feeding.
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Unio (Fresh Water Mussel):
It is found in rivers, lakes and ponds. The animal is omnivorous feeding on microscopic organisms. Its larva lives as parasite on fish. Its shell consists of two halves, called the valves. A whitish elevation in each valve is called umbo.
The lines of growth indicate the age of the individual. There are present two siphons posteriorly. It acts as scavenger and keeps water clean. Its shell yields an excellent quantity of lime the shells of the fresh water mussels are used in the manufacture of buttons.
Pila (Apple snail):
It inhabits ponds, paddy fields, sometimes streams and rivers It is chiefly hetmvorous and feeds on aquatic plants like Pistia and Valusnena. It leads an amohibious life respiring by means of gill in water and by a pulmonary sac an and. Thus it is adapted for both aquatic and terrestrial life.
The mouth or aperture of the shell is closed by a flat and oval plate, the operculum. Pila has osphradium which is meant for testing chemical and physical nature of water. The buccal cavity of Pila contains a rasping organ, the radula, with transverse rows of teeth for cutting the grasses.
Limax (Grey Slug):
It is terrestrial and is abundantly found in gardens, cultivated lands and over damp soil. It is nocturnal and herbivorous. Shell is internal. It is a hermaphrodite animal. The slug is a plant pest. It damages seedlings, tender shoots and leaves.
Sepia (Cuttle fish):
It is a marine and a good swimmer. In male the left arm is spoon shaped and is called hectocotylized which is used to transfer sperms into the female. The shell is internal. A pear-shaped ink-sac containing the ink-like fluid is present.
When the animal is attacked ink-like fluid is ejected through the funnel to form a smoke cloud. Sepia is edible. Sepia ink obtained from this animal is used by artists. The shell of sepia is used as a source of calcium for pet birds.
Lolieo (Squid):
Like Sepia, it is also found in the warm seas. It also ejects a dark ink to form a smoke cloud to escape from .the enemy. Its body resembles that of Sepia in form except that it is narrower than that of Sepia. It also has internal shell Largest.living invertebrate is giant squid (Architeuthis) up to 18 meters long. LxAigo is edible. The giant squid has the largest eye in the animal kingdom.
Octovus (Devil fish):
It is also marine. It kills its prey with poisonous saliva. One of the arms of male is spoon shaped and is called hectocotylized which is used to transfer sperms into the female. The shell is absent. Octopus ejects an inky fluid in water and forms a screen for defence from its enemies. Octopus can change its colour.
