152 MADRAS FISHERIES BULLETIN VOL. XIV, 



It is interesting to learn that the scaphopods were for long 

 mistaken by naturalists for tube-forming annelids, so close is the 

 superficial resemblance both of their tubes and of their head 

 filaments with those of certain of these marine worms, 



CLASS IV— LAMELLIBRANCHIA (PELECYPODA). 

 The most conspicuous characteristic of this class is that the 

 shell is in two parts or V(7lvcs, hence the convenient semi-popular 

 term of bivalves often applied to them. The mantle consists of 

 two lobes, a right and a left, corresponding to the two valves and 

 enveloping • the body between them. A head is virtually absent, 

 and no radula or teeth are present, whereas the foot, frequently 

 developed into a narrow axe-shaped organ, is often of considerable 

 size and of great mobility and strength in sand-bnrrowing species. 

 The breathing organs are developed into plate-shaped gills, 

 varying greatly in structure ; these differences are now made use 

 of in the classification of the group. The majority of bivalves 

 pass their lives in burrows in sand or mud; others anchor 

 themselves by a cable of fine threads, the byssus ; in some one 

 valve is cemented to rocks; a few bore into wood and stone and 

 others are so exceptionally active that they swim through the 

 water by alternately opening and shutting their valves. 



Their food consists of minute organisms, animal and vegetable, 

 swept within the cavity enclosed by the mantle lobes by the 

 rythmic lashing of microscopic threads or cilia lining the interior- 

 These particles are intercepted by the filter-like structure of the 

 gills and wafted to the mouth by other cilia. 



Before describing some of the common Indian bivalves, an 

 explanation of certain technical terms is necessary for the ready 

 understanding of the subject If an ordinary bivalve such as a 

 clam [inatti, Tamil) or a mussel {kallikai or pachchiidli, Tamil) 

 be taken in the hand and held in such position that the hinge is 

 uppermost and the end at which the mouth is situated is directed 

 forwards, we are holding the shell with its dorsal side uppermost, 

 its ventral side down, the anterior end turned forwards and the 

 posterior end towards the person holding the shell ; in this position 

 the valve on the right is the right valve, the other the left one. 

 The elastic pad joining the two valves at the hinge is the ligai/ieiit 

 and the fine skin on the surface of the shell is the pcriostracuin. 



