THE VARIED ROLES OF SNAILS - National Universities Commission

sculptural defenses against crushing may be of greater
importance to open surface dwelling species compared to
sand dweller.
For the lovers of beauty, shells of gastropods (fig. 6) have
always offered a wide variety of opportunities for artistic
ingenuity, producing many gift items. Some gastropods
have striking colour patterns showing a good deal of
contrasts. They may occur in the shell, foot, mantle or eys.
Some are understandably camouflage and warning features
and are distinctive for each species of one genus. Thus they
are often used for species recognition. Most pulmonate
gastropods have dull coloured shells without patterns. They
have constellations in addition to their species specific
micro-sculptures.
Using molecular biological studies, it was found that the
controls of colour patterns are located in several gene loci.
It has been suggested that shell patterns are caused by
deposition of shell pigments in the primitive molluscs.
These pigments are believed to be derived from waste
metabolic products removed from living tissues by
incorporating them into shells. The fact that the substances
are laid down in bands, spots, strips and other intricate
patterns of constant nature indicates that the disposal of
these wastes may definitely be under genetic control and so
have adaptive value.
Gastropod shells have three major layers – the inner or
Ostracum, the middle which is relatively thick is the
Calcareous while the outer or Periostracum is thin and
transparent. The Periostracum is made of chitin like protein
material called Concholin. This protein is secreted by the
mantle. The Ostracum is also known as the nacreous layer
23