I68 MADRAS FISHERIES BULLETIN VOL. XIV, 



when adult they usually desert this habit and lie free, resting upon 

 the right or convex valve. They have uncommonly active habits 

 for Lamellibranchs, especially when quite young, when some 

 species dart through the water by the alternate and rapid opening 

 and shutting of their valves. But even the adults have considerable 

 power of shifting their position in this way. Probably correlated 

 with this unusual activity, is the presence of highly developed 

 " eyes " upon the mantle edge. They are placed at the end of 

 short tentacles at irregular intervals among the long tentacular 

 filaments margining the "gape" of the shell. They are always 

 more numerous on the left or superior lobe ; these eyes are brilliant 

 iridescent green in colour, very conspicuous and arresting when 

 the animal is watched alive. In structure they are as complicated 

 and perfect in optical design as the human eye, with a cornea, 

 crystalline lens and retina of complex structure together with a 

 well-developed optical nerve. There can be no question as to the 

 functional value of such eyes ; they are made to see with and 

 are valuable in warning their owner of the approach of danger 

 independently of touch. 



The colours of scallop shells are usually bright with red and 

 brown and even canary yellow, but these are believed to have no 

 special significance. In the days of the Crusades a scallop shell 

 was the badge of pilgrims returning from Palestine. 



Very much finer than the small pectens met with in shallow 

 water, is the lovely porcellanous A/uussimn picuroiiectes, met with 

 in deeper water in the Bay of Bengal off the Ganjam and 

 Orissa coast. Unlike the pectens, this shell is smooth externally, 

 highly polished and handsomely decorated with reddish brown. 

 The dredge is necessary to procure it. 



The Thorny-oysters {Spondylidac) resemble the scallops in 

 anatomy, even to the presence of eyes bordering the mantle, but 

 they have abandoned a free life completely. The valves are both 

 ornamented with long spines and those of the lower (right) valve 

 become cemented more or less extensively to a rock or boulder. 

 Spondylus is a fairly common shell on rocky ground and among 

 coral reefs, where its brilliant red or yellow valves, decorated with 

 radial ribbing of long spines are conspicuous. It relies upon 

 quickness of sight and upon the strength and spinous defences of 

 its shell to defy enemies. That its valves may not be wrenched 



