132 SPOLTA ZEYLANICA. 



to (listinctions of sex, but are to some extent correlated with indivi- 

 dual variations in the black pigmentation of the mantle. In some 

 cases the mantle round the adductor muscle up to the apex of the 

 median hinge lobe is intensely black, the free peripheral portion 

 being destitute of the black pigment, unless there happens to be a 

 peripheral mantle pearl present, in which case, after removal of the 

 pearl, the pigmented sac in which it was contained shows up clearly 

 in the middle of the colourless tract. The dwarfed oysters of 

 Kakkaimunai were noted in general to be pale throughout, often 

 very little pigment being present in the mantle. Occasionally the 

 gonad has a dark smoky brown tint ; this was first observed in March 

 in an individual which proved to be a female. 



In both sexes the gonad is sniooth externally, but internally a 

 tine sacculation may be seen witli a lens. Frequently, when the 

 sacculation is coarse and even visible without a lens, the individual 

 turns out to be a male ; when the sacculation is so fine as to give a 

 minutely spotted appearance when viewed witli a lens, the indivi- 

 dual maybe a female; but this distinction is not absolute, and the 

 only ultimate criterion is afforded by microscopic examination. 

 Out of twenty individuals thus examined in March (1908), nine 

 were male, eleven female. One of the Kakkaimunai oysters, though 

 of small size, 4f by 4| inches, gave an impression of being old. 

 and was in fact a mature female. 



If an incision be made into the gonad of a mature female, a 

 quantity of minute brown- coloured ova will stream out ; but in a 

 mature or sub mature male the contents are viscid. 



The spermatozoa, as they appeared at this time, had round heads 

 and very delicate tails (fig. 1). They moved, on the addition of 

 sea water, in a jerky, spasmodic manner. The morulas, from which 

 they originated, had in very many instances not yet become disso- 

 ciated, and in these cases the long tails could be seen, under high 

 magnification, radiating on all sides from a mass of heads. Often 

 two or several spermatozoa were seen with heads still agglutinated 

 together (fig. 2). By way of comparison tlie ripe speiiuatozoa of a 

 species of Cardium, common at Niroddumunai (where it is called 

 " pakku maddi," this term being differently applied a( Koddiyar), 

 were sketched. These have elongate heads with hooked extremity 

 and broad base, from which the tail proceeds (fig. 4). I have not 

 yet had an opportunity to study the finer structure of the reproduc- 

 tive organs by means of sections. 



The observations described above lead to the conclusion that the 

 window-pane oysters living in lake Tamblegam in the first quarter 

 of 1908 displayed synchronous maturation of the gonads ; and that 

 Placuna is a periodic spawner, or at any rate does not become 

 continuously reproductive until a late age, in contrast with the 

 chronic maturity of other burrowing bivalves, such as Venus, 

 Cardium, and Area. 



