124 MAJDRAS FISHERIES BULLETIN [VOL. XVI, 



running to all parts of the body, but chiefly to the muscles and the 

 mantle edge, for active co-operation has to be maintained by these 

 organs; if the mantle filaments sense danger, they signal to the 

 ganglia and these transmit an impulse to those muscles to be 

 actuated in defence — usually the retraction of the mantle edge, and 

 the contraction of the adductor whereby the shell is closed. 



The sexes are separate, but no outward differences are dis- 

 cernible in the shells of the two sexes. The gonads or reproductive 

 glands consist of myriads of glandular tubules enveloping the 

 digestive gland and the alimentary canal. In the males the gonads 

 when ripe give a pale yellow tint to the surface of the visceral 

 mass ; in the female the colour is usually deeper, inclining to 

 orange. As the products ripen they pass into larger ducts and 

 these in turn empty into two main tubes communicating with the 

 exterior, one on each side near the anterior ends of the gills. 



Ova and spermatozoa are poured out by each sex haphazard 

 into the sea, where the ova have to chance fertilization. Probably 

 there is not such waste as one might infer, for these oysters form 

 beds counted by the million and the emission of either product 

 acts, as I have actually observed, as a direct stimulus to those of 

 the opposite sex that have not as yet emitted. April May and 

 September October are the maximal spawning seasons, but like 

 the edible oyster {Ostrea virginiand) of Indian back-waters, some 

 individuals can usually be found in a ripe condition at almost 

 any time. 



The ova when emitted are flask-shaped, the neck forming the 

 micropyle. After a spermatozoan has forced entrance into an 

 ovum through the micropyle, thereby effecting fertilization, the 

 latter becomes rounded, and soon begins to segment. Segmentation 

 is unequal. A trochophore form similar to that familiar to us in 

 many allied molluscs, as well as among the polychaet worms, con- 

 stitutes the first larval stage ; first appears a zonal and pre-oral 

 band of short cilia, followed a little later by the development of a 

 long apical tuft and a small posterior patch. By the 20th hour this 

 larva is fully formed and already shows a short enteron. By the 

 third day at latest, probably somewhat earlier when conditions are 

 favourable (the present results are based upon observations which 

 1 made on larvae living under artificial conditions in glass bowls), 

 the second larval stage, the veliger, is attained. In this a minute 

 ovoid embryonic shell has appeared, colourless, transparent ami 



