88 OKHAMANDAL MARINE ZOOLOGY REPORT 



stalk eventually functions as a micropyle. The vitelline membrane enclosing the 

 granular vitellus is slightly vacuolated ; the nucleus is large and very granular and 

 often contains two distinct nucleoli. 



The spermatozoa arise by a similar proliferation of the germinal epithelium in the 

 males; in maturing individuals the simple nature of each of the alveoli is masked 

 by the presence of large quantities of unripe spermatozoa which obscure and practically 

 obliterate the lumen. The individual spermatozoa are very minute and of the typical 

 form seen in Lamellibrunchs, each having a pear-shaped head, clear and highly 

 rcfractile, with a long, delicate flagellum proceeding from the broader end. 



The spawning season of Placuna appears to coincide with the onset of the north- 

 east monsoon both in Ceylon and the Gulf of Kutch, the two localities where I have 

 had opportunities to examine large numbers of individuals of various sizes. Thus in 

 January, 1906, I found oysters of 1'5 centimetres diameter about three months old in 

 Rann Bay on the Okliamandal coast, while the young seen in Tampalakam Bay, 

 Ceylon, in May, 1905, being 4 cms. in diameter, were approximately six to seven 

 months old ages which give the spawning season as the month of October in the 

 preceding year. 



On a priori grounds this is what might be expected. The month of October 

 witnesses a great change in weather on the Indian coast, with a concurrent 

 variation in the specific gravity of the water on the Placuna banks. In Ceylon this 

 is specially emphatic. October is pre-eminently a rainy mouth in the Tampalakam 

 district ; the inHux of fresh water into the bay which results lowers the specific gravity 

 and supplies the needed stimulus required by the window-pane oysters to bring on 

 widespread emission of the genital products. 



THE SIGNIFICANCE OF THE ASYMMETRY AND OTHER MAJOR 

 PECULIARITIES OF PLACUNA PLACENTA. 



The most striking and outstanding features which characterise the organisation of 

 Placuna are the marked asymmetry affecting almost every organ, and the archaic 

 characters retained by many of them. 



Regarding asymmetry, F. J. H. Lacaze-Duthiers, working a half century ago on 

 the anatomy of Anomia epkippium, belonging to a closely related genus, " was struck 

 at the outset of the investigation with the importance and preponderance which the 

 right mantle assumed over the left." A similar remark would apply with equal force 

 to Placuna placenta. 



In this species asymmetry affects practically the whole organisation. It is carried 

 so far in the case of the reproductive system that this may be said to lie almost 

 wholly within the right mantle. 



The auricles are markedly asymmetric in calibre ; the sinuses of the two mantle 



