No. 6 (I92n COMMON MOLLUSCS OF SOUTH INDLA. 99 



I am under great obligation for the care they have exercised in 

 carrying out my ideas, and for the skill shown in the actual 

 drawings. The exigencies of printing have, I fear, obscured in 

 too many cases the excellence of their work. 



GENERAL. 



The conmionest objects on the sea-beach are the shells of 

 those soft-bodied animals called by zoologists molliisca and popu- 

 larly known as shellfish. These names appear at first sight to be 

 contradictory, for the scientific term signifies that the animals are 

 soft-bodied, while the popular name implies that they are 

 encased in hard protective coverings or shells. Combining the 

 two we get a fairly clear idea of their characteristics — they are 

 soft-bodied animals protected by an outer casing usually hard, 

 composed of some form of limy or calcareous material, and 

 without any internal skeleton except in very exceptional instances. 



The shells of molluscs are extremely diversified in shape and 

 colouring. Each may consist of two parts or valves as in the 

 oyster and the mussel {Bivalves, Lai/iellihranehia or Pelecypoda), of 

 a series of plates as in C!iiton, the coat-of-mail shell {Auiphiiieiit'a), 

 cr of a single piece, in most cases spirally twisted (Gastropoda) ; 

 finally come the highest of the group, the Cephalopoda, comprising 

 the most active and intelligent of all the mollusca, the octopus-, 

 cuttlefishes, and squids, together with the nautilus and the whole 

 host of fossil ammonites. A division (Seaphopoda) has to be made 

 to receive the elephant-tusk shells, a small number of peculiar 

 forms with long tapered tubular shells well described by their 

 vulgar name. 



Except in bivalves the body of molluscs can be made out 

 usually to consist of three regions — an anterior part marked out 

 as a distinct head, bearing the mouth, tentacles and the chief 

 organs of sense ; behind this is a swollen dorsal mass, the visceral 

 sac, containing the intestine, liver, and reproductive organs. The 

 integument of this mass is the mantle and this is generally 

 disposed in a fold reflected over the back of the animal. The 

 outer surface of the mantle secretes the shell ; in bivalves the 

 mantle is double, formed a free fold or flap on either side of the 

 body, hence originating the two valves or half shells characteristic 

 of these molluscs. On the ventral side of the body below the 

 visceral mass is a large muscular organ, the foot, flattened and 



