1922] 



MADRAS PEARL FISHERIES 



125 



structureless. From beyond the widely opened edges of this 

 minute bivalve shell protiudes a large lobe, the velum, fringed 





sillSiii 



iWiii pi 



Fig 5. Larvre of the Indian pearl-oyster. A. and B. trochophore stages ; 

 C. earliest form of the veliger stage. Very highly magnified. 



with cilia, a modification of the pre-oral region of the trochophore 

 larva. Soon various organs begin to make their appearance as 

 shown in figure 6. In these larvae, as yet free-swimming, the need 

 for sense organs is greater than in the sedentary adult ; so we find 

 developed a pigmented spot, a rude eye, and an otocyst composed 

 of a tiny sac containing minute refractive granules. Developing 

 gill filaments appear, together with a bilobed digestive gland, and, 

 finally two adductor muscles, one at each end of the body. 

 Advanced larvae at this stage constitute the " spat " that replenishes 

 our pearl banks by means of a " spat-fall." The young oyster is 

 now ready to settle down upon the bottom and begin its sedentary 



