1922] 



MADRAS PEARL FISHERIES 



127 



intermediate region lined by nacre. Eventually the embryonic shell 

 becomes imbedded in the adult shell and ceases to be distinguish- 

 able. At an early stage the byssal sinus appears as a tiny bay at 

 the anterior side of the shell. Even in the free-swimming stage the 

 foot had appeared and as soon as the spat settles, this organ and its 



KlG. 7. Pearl oyster spat in progressive stages. A. form at time of attachment ; 

 B. shows the beginning of the prismatic layer ; C. and D. very young forms 

 of the immature oyster, the byssal sinus already distinguishable. In D. 

 the gill filaments are visible. 



byssal gland begin to function by producing tiny byssal threads to 

 serve as attachment cables. For some days after settling the spat is 

 actively locomotive, continually throwing off its byssus and mov- 

 ing to a new foothold. Little by little this activity slackens but 

 under uncomfortable conditions even the adult may repeatedly cast 

 off and renew its byssus, crawling slowly to a new position in the 

 interval. I have seen this happen up to eight times in fourteen 

 days, in the case of a marked oyster kept under observation in an 

 aquarium tank. 



It is of great practical importance to be able to distinguish the 

 true pearl oyster spat from that of its near relative, Avicula vexillum, 

 called most appropriately "False-spat" in our records. Time 



