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  Characteristics of the mollusca, Biology tutorial

Features of molluscan classes:

Class Scaphopoda (tooth-shells; tusk-shells):

The body is lengthened almost cylindrical and bilaterally symmetrical. Shell is tubular, tusk-shaped and open at both ends. They have short foot. They do not have a heart. Head is reduced with no eyes but contains bundles of ciliated prehensile tentacles known as captacula utilized for feeding. Only approx 900 species; like Cadulus sp., Dentalium sp., etc.

Class Bivalvia (Lamellibranchiata; Pelecypoda; bivalves; clams; oysters; scallops):

Name Bivalvia comes from their having two shell valves. There are approx 20,000 species of bivalves. They are laterally compressed molluscs entirely surrounded within 2 shell valves. They are comparatively sedentary, numerous being cemented or else joined to substratum, and; head is greatly decreased. Missing sense organs of head may be replaced by tentacles, and in few, eyes placed around margins of mantle. Their extremely important gills are utilized for filter-feeding and respiration.

Characteristics:

They are all aquatic and are found in freshwater and the sea. They are bilaterally symmetrical. Their body is laterally compressed and enclosed in a shell that develops as two large plate-like valves hence the name Bivalvia. The head is rudimentary (greatly reduced). They lack tentacles, eyes and radula. The foot is wedge/hatchet/tongue-shaped and can be protruded for ploughing into soft deposits. They are mainly filter feeders. Fertilization is external. A glochidium is the final larval stage of somefreshwater bivalves. E.g. Mytilus (mussel); Ostrea (oyster), Pecten, Anodonta, Ensis, Teredo, etc.

Class Gastropoda (belly-footed; belly foots):

The name Gastropoda explains group in which broad foot occupies most of the underside. Stomach and digestive gland sit on top of muscular foot. Class Gastropoda is largest molluscan class with more than 75,000 living species, and long fossil record. Gastropods illustrate significant adaptive radiation and are create in all types of marine environment, both at surface and on bottom, and in freshwater and on dry land. Usually, gastropods have well developed head with tentacles and the asymmetrical body. Shell is adapted as a house in which animal can retreat rather than as simple protective shield. Considering wide variety of habitats that gastropods have invaded, they can be regarded as most successful group of molluscs.

Torsion:

All gastropods share phenomenon in embryonic development (embryology). Torsion is anti-clockwise rotation of visceral hump/mass by 180 degrees, bringing mantle cavity from posterior to anterior position. Torsion is caused by differential development of two pedal retractor muscles. Benefit of anterior position of mantle cavity might comprise:

Provision of accommodation for well-developed head on retraction of gastropod in its shell

Chemoreceptors (osphradia) are brought to anterior end of animal so that it can sense water ahead in movement.

Gastropods have overcome dumping of excretory products over head by modifying path of exhalent current by:

  • Development of the slit in shell by some by which exhalent current can leave.
  • Secondary detorsion by 90 degrees in some of them, so that mantle cavity lies on right side of body and anus becomes posterior.

Coiling:

Most gastropods are snails with the helically coiled shell and visceral mass. Helical coiling results from unequal/asymmetrical development. Most snail shells are in form of right-handed coil that leaves less room for organs of right side; several snails don't have right gill, right excretory organ and right auricle.

The early coiled shell was planospiral that is bilaterally symmetrical with each spiral situated outside preceding one, and in same plane that resembles the coiled hose lying flat on ground.

Reduction in shell aperture which resulted in limitation of space within margin of mantle may have accounted for reduction of gills, retractor muscles and nephridia to the single pair that is maximum number in any gastropod. The drawback of planospiral shell is that it is not compact and thus cumbersome. This problem was solved with evolution of asymmetrical coiling, where coils were laid down around collumella with each coil lying beneath preceding one. This kind of shell is comparatively compact and may have globular shape, despite its length. New kind of shell couldn't be carried as old planospiral shell as all weight would hang on one side of body.

Other characteristics of the gastropods:

They are primarily marine but some happen in freshwater and terrestrial habitats. Possess distinct and well-developed head bearing tentacles and eyes. They contain well-developed radula and have flat muscular ventral foot used for creeping. They may or may not have shell and if present, single unit. E.g. giant West African land snail, Limax (slug), Patella (limpet), Haliotis (abalone), Achatina, etc.

Class Cephalopoda (head-footed; nautilids, squids, cuttlefish and octopuses)

The name of class signifies head-footed, as the foot is closely related with head. Like in gastropods, the shell has been decreased in varying degrees. Nautilus is classified by large external coiled shell, while, squids have thin non-calcified cellophane-like vestige of shell embedded in mantle; cuttlefishes have calcified shell which is also reduced and embedded in mantle; octopuses do not have shell at all. Reduction of shell is connected with more active swimming lifestyle; it is a buoyancy device in cuttlefishes. The nautiloids are poor swimmers with their massive, many-chambered shell. Like in gastropods, cephalopods are lengthened along dorso-ventral plane, but due to their swimming lifestyle, ancestrally ventral surface has turned into anterior end and dorsal (visceral hump/mass) has developed into posterior end, functionally. Several cephalopod characteristics are directly or indirectly associated to their active life and likewise higher metabolic rate. Highly developed eyes; complex nervous system and behavior; chromatophores and ink gland.

Other features/Characteristics:

They are completely marine animals, they are bilaterally symmetrical. Long axis of body is dorso-ventral. Cephalopods particularly squids have evolved several similarities to vertebrates e.g. squids have internal cartilaginous support analogous to vertebrate skeleton; they have a cartilaginous brain case. Shell may be huge and external, reduced and internal or completely absent. Radula is well developed and additionally, there is pair of horny mandibles which form beak. E.g. Octopus, Loligo (squid), Argonauta, Nautilus etc. Cephalopods are now signified by only approx 400 species compared with more than 10,000 different fossil forms. There are 3 subclasses of cephalopods, one fossil and other two extant.

i) Subclass Ammonoidea: These are fossil forms with coiled external shells and complex septa and sutures.

ii) Subclass Nautiloidea: External shells that may be coiled or straight; shell sutures simple; many slender suckerless tentacles; 2 pairs of gills; 2 pairs of nephridia. Nautilus is only surviving member of the group.

iii) Subclass Coleoidea: Shell internal or absent; some tentacles with suckers; one pair of gills; one pair of nephridia. Five orders: 1 extinct and 4 extant. like Sepia, Sepiola, Spirula, Idiosepius, Rossia (Order Sepioidea); Squids: Loligo (Order Teuthoidea); Vampyrotheutis (Order Vampyromorpha); the octopus: Argonauta; Eledone, Eledonella etc. (Order Octopoda: 8 legs)

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