Shell in Mollusca.

Shell in Mollusca:

Shell in Mollusca is usually external, sometimes internal. It is derived from the mantle of the veliger larva. It may be univalved, bivalved, cone-shaped or spirally coiled and sometimes in a linear row of eight valves.They are characterised by having, as a rule, a protective exoskeleton in the form of a shell. In the adult, the shell in Mollusca is represented by the smallest whole at the apex of the shell. They are differentiated into Structure of shell and Variety of shells.

[I] Structure of shell:

The structure of shell is composed of three layers. The outermost pigmented layer called Periostracum. The periostracum is made of horny organic substance, Conchiolin. It is secreted by the edge of the mantle. Below the outer layer is the Prismatic layer, also secreted by mantle edge.It is made up of crystalline calcareous plates running vertically. The innermost layer is called conchiolin and calcareous plates running alternatively and longitudinally. It is secreted by the whole outer surface of the mantle and presents a lustrous surface so-called "mother of snail".

[II]Variety 0f shell:

A large variety of shell is found in Mollusca. Shell of one representative type of each class is discussed below. Some of them are

1. Chiton:

It is made up of eight transverse, overlapping, calcareous plates or valve, arranged in a longitudinal row. These shells form the most characteristic feature of Chitons. Polyplacophora is derived from the nature of this shell. These shell plates are moveable upon one another allow the animal to roll up like a woodlouse.Each shell plate is made up of two distinct layers. Tugmentus or upper layer consists of an organic conchiolin matrix and lower thicker and a dense layer called articular mentum consists of calcium carbonate only. First (cephalic) and eight (anal) shell plates are hemispherical shell plate are somewhat rectangular and often keeled mid-dorsally.The posterior edge of each plate overlaps anterior edge (wing) of the next behind.

2. Dentalium:

Dentalium is commonly known as tusk shell.It is shaped like a trumpet or elephant’s tusk, hence often called “tusk shells”. Shell is external in Dentalium. The body is cylindrical tubular, slightly curved bilaterally symmetrical tapering and enclosed in a tubular shell open at both the ends.During life, the shell is buried obliquely in the mud with wider anterior end lying deepest and narrow posterior end of apex projecting above the surface of the mud. Mantle folds are fused ventrally to form enclosing the body. Shell of D.octogonum is snowy-white with 8 grooves running down the entire length, that of D.formosum is mottled with shades of dark red. There is no operculum and shell grows at the larger anterior end. The scaphopod shell differs from that of other Mollusca in being unchambered and open at both ends and that of tubiculous annelids in heaving 3 instead of 2 layers.

Some ecological notes on Dentalium is that

1.The head is vestigial, bearing the mouth which is surrounded by a circuit of retractile tentacles, the captacula with sucker-like ends.

2. Well, developed radula is present.
3. Anus lies at the base of the foot.
4. Gills are absent. Respiration by transverse folds in the lining of the mantle.
5. The nervous system simple and consists of cerebral, pleural and pedal ganglia.
6. Eyes are absent. Otocysts present.
7. Sexes are separated.

3. Cypraea.

They are also known as cowries. Shell of Cypraea is oval, rounded on the top and flattened beneath and consists mostly of a large whole. The shell opening is long, and narrow toothed on both sides and channelled at each end. Their extremely smooth, highly polished and variously coloured shell make very conspicuous and prized by the collectors

4. Mytilus:

Shell is equivalve and wedge-shaped being pointed in front and rounded behind. Two shell valves are united anterodorsally by a conchiolin and are brown, tough, elastic and non-cancerous . Umbo, a whitish knob-like swelling in each valve lies anteriorly. Umbo is the thickest and oldest portion of the shell. Lines or rings of growth run around umbo as the centre and run parallel to the free margin of the shell.

5. Sepia:

Sepia is commonly known as cuttlefish. It is marine form found in the shallow waters and has a cosmopolitan distribution. They are flat, broad, oval in shape bilaterally symmetrical represented by phragmocone with a broader and rounded oral end, called pro-ostracum and a narrow, pointed aboral end called rostrum projecting into a spine.Shell is dead and composed of calcareous rather than horny matter.

They body is divisible into head, neck and trunk. Head bears a pair of large eyes and five pairs of arms, surrounding the mouth. Of the five pairs of arms, four pairs are short and stout bearing four longitudinal rows of sucker on the inner flat surface. The fifth pair of arms is known as tentacles which are comparatively longer and narrow and provided with suckers only towards their free ends. The trunk is elongated and shield-shaped. Shell is internal and enclosed in a shell sac in the mantle on the upper surface. Chromatophores are present in the deeper layers of integument over the entire surface. A pair of plume shaped ctenidia or gills, one each side of the mantle cavity performs the respiratory function. A pair shaped ink sac lies over the posterior ventral surface. Sexes are separated . In the male, the left fourth arm is hectocotylized.

Reference:

Aggarwal Sarita. A Text Book of Biology,New Delhi.: Madhuban Educational Books, 2011.

Bhamrah, H.S., and Kavita, Juneja. A Text Book of Invertebrates, New Delhi: Anmol Publications Pvt Ltd, 2011.

Jordan E.L. and P. S., Verma. Invertebrate Zoology, New Delhi,: S. Chand and Company Pvt. Ltd., 2011.

Kotpal, R. L.,Modern Text Book of Zoology: Invertebrates, New Delhi, India: Rastogi Publications,2011.

https://en.wikipedia.org/wiki/Mollusc_shell