TOO MADRAS FISHERIES BUI.LETlN vOL. XIV, 



adapted for crawling in the gastropods and chitons ; hatchet shaped 

 in bivalves; in cephalopods it is split np into a number of 

 mobile arms beset with suckers. 



The mollusca are a difficult subject for the evolutionist ; they 

 appear in groups as generally distinct and fully differentiated in 

 the lower Palaeozoic strata as at the present day ; gastropods and 

 lamellibranchs are found in the Cambrian and the remaining 

 classes in the Ordovician or Lower Silurian. There is no geologi- 

 cal sequence to help us to a decision ; the larval history of some 

 species gives a hint, for a characteristic larval form of many 

 molluscs is a tiny globular body furnished with a circlet of cilia 

 round the middle ; the lashing of the cilia causes it to spin through 

 the water. This larva is termed a trochosphere and is practically 

 identical with the larvae of many of the bristle worms (poly- 

 chaeta). Excepting the lamellibranchs, all other classes (with 

 the usual qualihcation of " exception " that meets us at every turn 

 in the study of zoology) possess a ribbon-shaped tongue, or radiila, 

 set with rows of teeth ; these molluscs generally possess a definite 

 head, bearing eyes and other sensory organs. If the larval history 

 of molluscs points to an ancestry among those marine worms that 

 are typically furnished with a well-marked head and in several 

 families with a gullet armed with a series of horny teeth, we may 

 conclude that those molluscs with a head region and a radula are 

 more akin to the ancestral form than the lamellibranchs, which 

 have diverged in order to fit themselves for a sedentary life. This 

 change has led to the loss of a definite head region and of the 

 tooth-set radula. In specializing, they have become degenerate in 

 several ways. 



CLASS I.— AMPHINEURA. 



The Chitons or Coat-of-mail shells are the only members of 

 this class that need concern us. Like the gastropods proper the foot 

 is well developed as a crawling organ, flattened and extending the 

 whole length of the elliptical body. The species most common in 

 Indian waters gro.v to a length of l^ to 2 inches. They live 

 among rocks, generally between tide-marks and often may be seen 

 adhering to the sides of rock pools at Cannanore and on the shore 

 reef at Rameswaram. Another favourite haunt is the eastern reef- 

 flat of Krusadai Island (Pamban) where they are often found 

 clinging to the under side of boulders poised on others. 



