Mollusca: General Characters and Classification
'Mollusca' is derived from Latin word mollis or molluscs which mean soft bodied. This term was first applied by Aristotle to the cuttle-fish of the Aegean Sea.
Definition:
Mollusca which include clams, snails, slugs, squids, octopods and nautili are triploblastic, bilaterally symmetrical animals with anus and coelom and without segmentation. They usually have shell and a characteristic ventral muscular foot. There are 80,000 known living species in this phylum.
Derivation of Name:
In Latin, mollusca is the name of a softnut with a thin shell, referring to the bivalve shell and the soft bodied animal within the shell. The term Molluscs are among the most abundant of all animals. In number of species, the Mollusca is the second largest phylum after Arthopoda. It is not possible to assess accurately 'the total of the known species. But probably 100,000 living and a good number of fossil species (35,000) exist, as compared with ten times as many insects but only half as many vertebrates. Three quarters (80,000 species) of the Mollusca the are gastropodes with about 1,700 genera. Molluscs have no uniform plan as well as no specific shape. In an evolutionary sense they are plastic material as the out lines of the body are freely altered as new habitats are acquired and new structures are needed. Most of them are slow moving and confined to rather special habitats.
Evolutionary Characters:
Molluscans exhibit few evolutionary characteristics which are,
1. Some molluscs are herbivorous while others are carnivorous. The digestive system is largely extracellular.
2. They mostly have ganglionated nervous system and the ganglia have a tendency to become concentrated at the anterior end. The nephridial wall tends to become evaginated and folded to effect an increase in the surface area for tubular secretion of waste picked up from circulatory blood. The gonads have lost their primitive association with the pericardial cavity and have mounted on special axis to the outside. 3.
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General Characters:
Molluscs range from limpets clinging to the rocks, to snails which crawl or dig or swim, to bivalves which anchor, burrow or bore, to cephalopods which torpedo through water or lurk watchfully on the bottom. They penetrate all habitats: the abysses of the sea, coral reefs, mudflats, deserts and forests, rivers, lakes and underground. They may be hidden as rasites ir the interior of other animals. They feed on every possible food and vary in size from giant squids and clams to little snails, a millimeter long. They form one of the most definitely characterized group of animals. They have atleast two characters 'radula and mantle' not found elsewhere.
1. Tissue-system grade of body organization.
2. Triploblastic, coelomate, unsegmented (except in Monoplacophora) and bilaterally symmetrical.
3. Body divisible into head, mantle, foot and visceral mass.
4 Shell, when present, usually univalve or bivalve, constituting an exoskeleton, internal in some.
5. Coelom reduced and represented mainly by pericardial cavity, gonadial cavity and kidney.
6. Digestive system complete with a digestive gland or liver (hepatopancreas); a rasping organ, the radula, usually present.
7. Circulatory system mainly of closed type, but some emptying into sinuses; heart with one or two auricles and one ventricle; blood with amoebocytes and haemocyanin. 8. Respiration direct or by gills or lungs or both.
9. Excretion by paired metanephridia (kidneys).
10. Nervous system of paired connectives and nerves. Ganglia usually form a circumenteric ring.
11. Sense organs include eyes, statocysts and receptors for touch, smell and taste.
12. Dioecious or monoecious; one or two gonads with gonoducts, opening into renal ducts or to exterior.
13. Fertilization external or internal; development direct or through free larval forms.
14. Terrestrial or aquatic (freshwater or marine).
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Classification:
Molluscs are classified into six classes according to their symmetry and the characters of food, shell, mantle, gills, nervous system, muscles and radula.
(Gr., monos, one+plax, plate+pherein, bearing)
1. Body bilaterally symmetrical, with dome-shaped mantle.
2. Flattened limpet-shaped shell with spirally coiled protoconch.
3. Foot broad and flat, with 8 pairs of pedal retractor muscles.
4. Five pairs of gills in pallial grooves.
5. Six pairs of nephridia, two of which are gonoducts.
6. Radula in a raddular sac; intestine much coiled.
7. Heart of two pairs of auricles and a single ventricle.
8. Nervous system with longitudinal pallial and pedal cords.
9. Internal segmentation.
10. Marine,
Example: Neopilina galatheae.
(Gr., amphi, both neuron, nerve)
1. Elongated body with reduced head.
2. Radula present.
3. Shell as 8 dorsal plates or as spicules.
4. Foot ventral, large, flat and muscular.
5. Non-ganglionated nerve ring around mouth with 2 pairs of interconnected nerve cord.
6. Fertilization external; larva trochophore.
7. Marine.
Subclass 1. Aplacophora
1. Body worm-like with a mantle but no shell and foot.
2. Calcareous spicules buried in cuticle.
3. Radula simple; mantle cavity posterior, some with a pair of bipectinate ctenidia. Chaetoderma.
Examples Neomenia, Nematomeina,
Subclass 2. Polyplacophora
1. Dorso-ventrally flattened body, small head (no eyes and tentacles); radula, mantle, foot and external gills present.
2. Mantle cavity posterior.
3. Shell as 8 calcareous dorsal plates.
Order 1. Lepidopleurina
1. Valves of shell without insertion plates.
2. Ctenidia a few and posterior.
Example: Lepidopleurus.
Order 2. Chitonida
1. Valves of shell with insertion plates.
2. Gills along whole length of mantle groove.
Examples: Chaetopleura,Chiton, Ischnochiton.
(Gr., scapha, boat+podos, foot)
1. Tusk-shells.
2. Body within a tubular shell, open at both ends.
3. No head; mouth with tentacles; no eyes.
4. Foot conical, radula present; no gills.
5. Kidneys paired; gonad single.
6. Dioecious; larva trochophore.
7. Marine.
Examples: Dentalium, Cadulus, Pulsellum.
(Gr., gaster, belly+podos, foot)
1. Snails and slugs.
2. Torsion (coiling) of body mass at sometime in development.
3. Well developed head with eyes and tentacles; radula present.
4. Foot large and flat.Shell present or absent; univalve and usually coiled. Mostly marine; some freshwater terrestrial.
(=Streptoneura)
1. Body mass torted.
2. Head with a single pair of tentacles.
3. Shell closed by an operculum borne on foot. Two ctenidia in mantle cavity situated anterior to the heart.
4. Nervous system streptoneurous with pleuro-visceral connectives twisted in a figure of 8. Sexes separate; gonad single; larvae trochophore or veliger.
5. Marine, freshwater or terrestrial.
Order 1. Archaeogastropoda (=Aspidobranchia)
1. One or two bipectinate ctenidia.
2. Two kidneys and heart with two auricles. Shell usually coiled. Nervous system not concentrated, with pedal cord.
3. Genital products conveyed to outside through right kidney.
4. Fertilization external.
Examples: Fissurella (key-hole limpet), Patella (limpet), Trochus (top shell), Acmars (limpet).
(=Pectinibranchia)
1. One auricle, one mono-pectinate ctenidium.one Siphon, operculum and penis present; osphradium developed.
2. Nervous system without pedal cords.
3. Fertilization internal; larva usually free-swimming veliger.
4. Mostly marine, some freshwater.
Examples: Pila (apple snail), Littorina (periwinkle), Crepidula (slipper shell),Cypraea (cowrie), Natica (star shell).
(Stenoglossa)
1. Shell with a short to very long siphonal canal.
2. Nervous system concentrated. Osphradium large.
3. Free-swimming veliger suppressed.
Examples:Nassarius,Buccinum (whelk), Murex, Oliva, Magilus,Malongena.
Subclass 2. Opisthobranchia
1. small without operculum or no shell.
2. Body mass torted or detorted.
3 Gills posterior to heart.
4. One one kidney, and one gonad.
5. Nervous system euthyneurous, i.c., without twisted pleuro-visceral loop.
6. Monoecious; larva veliger.
7. Exclusively marine.
Order 1. Cephalaspidea
1. Shell moderately developed.
2. Head with tentacular shield.
3. Lateral parapodial lobes prominent.
Examples: Acteon, Bulla.
Order 2. Anaspidea
1. Shell usually reduced or internal. 2. Well developed parapodial lobes.
3. Head with a pair of rhinophores.
Example: Aplysia (sea-hare).
Order 3. Pteropoda
1. Shell present or absent.
2. Parapodial fins for swimming.
3. With or without mantle cavity.
4. Head with a pair of rhinophores. Examples: Catolina, Clione, Corolla.
Order 4. Sacoglossa
1. Shell present or absent.
2. Pharynx suctorial. Spermduct closed.
Examples: Oxynoe, Elysia.
Order 5. Acochlidiaceae
1. Minute to small-sized; no shell; no gill.
2. Visceral mass separated from foot and covered with spicules. 3. Inhabit coarse sand.
Examples: Acochlidium, Unela.
Order 6. Notaspidea
1. Shell external or reduced and internal.
2. Mantle, but no mantle cavity.
3. Bipectinate osphradium on the right side.
Example: Pleurobranchus.
Order 7. Nudibranchia
1. No shell, gill, mantle cavity and osphradium.
2. Various dorsal outgrowths. Examples: Doris, Aeolis.
Order 8. Pyramidellacea
1. Shell typically spirally coiled.
2. An operculum but no gill and radula; proboscis long.
3. Semi-parasitic.
Example: Pyramidella.
Order 9. Philinoglossacea
1. Small naked snails without gill and head appendages.
2.Visceral mass separated from foot only by a groove.
Examples: Philinoglossa, Sapha.
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Order 10. Rhodopacea
1. Vermiform appendages. snails, without external.
2. Anus on right side of body. Example: Rhodope (Single genus).
Order 11. Onchidiacea
1. Slug-like without shell and with pulmonary sac.
2. Posteriorly located anus. Examples: Onchidium, Onchidella.
Order 12. Parasita
1. Endoparasitic in holothurians.
2. Shelled embryos.
Examples: Entoconcha, Thyonicola.
Subclass 3. Pulmonata
1. Detorted body mass.
2. With or without shell.
3. Mantle cavity or pulmonary sac with a pore on right side anteriorly.
4. Hermaphrodites; mostly freshwater or terrestrial, a few marine.
Order 1. Basommatophora
1. One pair of non invaginable tentacles.
2. Eyes at tentacular bases.
3. Male or female gonopores generally separate.
Examples: Siphonaria, Lymnaea, Planorbis.
Order 2. Stylommatophora
1. Two pairs of invaginable tentacles.
2. Second pair of tentacles with eyes at their tips.
3. Male and female gonopores usually united. Examples: Partula, Helix (land snail), Limax (slug).
(Bivalvia, Lamellibranchia)
(Gr., pelekus, batchet +podos, foot)
1. Body enclosed in a bivalve shell and laterally compressed.
2. No head, tentacles, eyes, jaws and radula. 3. Foot often hatchet-shaped and extending between mantle lobes.
4. Mostly filter-feeding.
5. Usually dioecious, veliger or glochidium larva.
6. Mostly marine, a few freshwater.
Order 1. Protobranchia
1. Gill filaments not folded.
2. Mouth placed at the base of muscular proboscides.
3. Stomach with style sac.
Examples: Nucula, Nuculina, Yoldia,Malletia.
Order 2. Filibranchia
1. Gill filaments reflexed but incompletely
fused.
2. Chitinous gastric shield in stomach developed.
3. Style sac with crystalline style.
Examples : Ostrea, Mytilus (mussel), Spondylus (edible oyster), Pecten (scallop).
Order 3. Eulamellibranchia
1. Gill filaments reflexed and fused completely to form tissue sheets.
2. Gills function for food gathering.
3. Style sac short.
Examples: Teredo (ship-worm), Ensis (razor clam), Unio, Anodonta, Lamellidens.
Order 4. Septibranchia
1. Gills absent.
2. Stomach lined by chitin; style sac reduced.
3. Marine.
Examples: Poromya, Cuspidaria.
1. Body elongated dorsoventrally.
2. Shell external, internal or absent.
3. Head distinct and large with well-developed eyes; foot as tentacles and siphon; radula present.
4. Dioecious; development direct.
5. Marine and free-swimming.
(=Tetrabranchia)
1. Shell external, coiled or straight; without complex sutures.
2. Recent species with many tentacles.
3. Two pairs of gills; two pairs of nephridia.
Example: Nautilus.suckerless
Subclass 2. Smmonoidea
1. Extinct.
2. Shell external and coiled with complex septa and sutures.
Example: Pachydiscus.
(=Dibranchia)
1. Shell internal or absent.
2. Tentacles a few with suckers.
3. One pair of gills, one pair of nephridia.
Order 1. Decapoda
Ten arms-two elongated and called tentacles, and eight short arms.
Examples: Loligo (squid), Sepia (cuttle-fish),Spirula (spiral shell).
Order 2. Octopoda
1. Body globular and without fins.
2. Eight equal arms.
Examples: Octopus, Argonauta.
(Reference: Modern Textbook of Zoology Invertebrates by R.L. kothapal Rastogi publication, Meerut, New Delhi)
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