No. 6 (1921) COMMON MOLLUSCS OF SOUTH INDIA 10/ 



The shell is often prettily marked — it varies very greatly — and in 

 old ones the surface is much corroded and pitted (mollusc shells are 

 normally protected against damage by a horny skin, the periostra- 

 ciim, or else by folds of the mantle wrapped over them. If the 

 periostracum be worn away, the limy shell beneath is liable to 

 corrosion, especially in shells often exposed by the receding tide). 



As in the Turbos, the operculum in the whole family of the 

 Nerites is stony. Usually, it is roughly semi-lunar in shape ; at one 

 end is a finger-like projection that hitches behind the columellar 

 lip and forms a locking device, giving additional security. A 

 peculiar little hermit crab (Coeiiobita i^ugosns) that frequents the 

 beach and lives almost entirely out of water, finds the empty shells 

 of Nerita just suited to his wants ; indeed he has adapted the shape 

 of his big claw so precisely to the form of the mouth of the shell 

 that when he shuts himself up inside, the big claw closes the 

 aperture as perfectly as did the operculum of the living shell. 



In the fresh-water Neritina the shell is thinner and the spire 

 still less evident ; finally in Septaria the spire is so completely lost 

 that the shell may easily be mistaken for that of a small narrow 

 limpet. Internally the columellar lip persists as a thin ledge and 

 there still remains a little operculum, too small to fit the aperture 

 and more or less embedded in the foot. 



Order 2.— Monotocardia. 



In these gastropods the heart has a single auricle only, and the 

 gill is single, with a single row of plates (monopectinate). The 

 body is greatly twisted and this results in the shell being frequently 

 drawn out into a long spire. There is practically no vestige of the 

 bilateral symmetry that can be traced in some organs in the 

 Diotocardia even when their shells are spirally coiled, as in the 

 Top-shells, The group is an exceedingly large and varied one and 

 includes some 76 families. Many are extremely specialized for life 

 under abnormal conditions ; some have adapted a life on the high 

 seas — pelagic — swimming or floating ; others have migrated to the 

 land and to fresh-water pools; a number have become parasites or 

 live as messmates (commensals) ; they may be carnivorous and even 

 possess poison glands, or they may be vegetarian in their diet ; 

 some move with considerable rapidity, others attach their shells to 

 rocks and simulate the appearance of certain tube-building worms. 



