126 



MADRAS FISHERIES BULLETIN 



[vol. XVI, 



existence. This may happen as early as the fifth day after fertili- 

 zation, but may be delayed considerably if conditions for the 

 change be unfavourable, as for example the prevalence of a strong 

 current or stormy weather. After settlement, the velum is retracted 

 and eventually absorbed and growth of the embryonic shell ceases. 

 In the course of a day or two, shell-growth of a different character, 

 marked out into a minute hexagonal pattern, is observable around 

 the margin of the valves; this is the first appearance of the 



Fig. 6. Later phases of the veliger stage, a ', anterior adductor muscle ; a", posterior 

 adductor ; e, eye-spot ; o, otocyst ; g, embryonic gills ; v, Velum; c ', enlarged 

 view of otocyst containing granules. 



prismatic shell-layer. Eventually growth produces such a form as 

 is seen in figure 7, B- The spat is now about one millimetre 

 across, and three regions are distinctly visible in the shell, 

 (a) the clear embryonic shell marked by delicate lines of growth, 

 and forming the hinge, (armed it should be noted with a number of 

 minute teeth) and the prominent umbo ; (/>) the now very extensive 

 prismatic region extending to the margin all round and gradually 

 producing the adult form of the shell, and (r: an internal and 



