258 CEYLON PEARL OYSTER REPORT. 



side. The most remarkable feature about the gonaducts is that each, just before its 

 external opening, is joined by the short and contracted duct of an ovoid vesicle 

 (Vs., figs. 1, 24, 25) which, in nearly all cases, is filled with ripe spermatozoa, and is 

 clearly a seminal vesicle, in which the ripe spermatozoa are stored up pending the 

 development of the ova. These seminal vesicles, which form very conspicuous objects 

 in sections, are lined by a well-defined, flattened, and as far as I could determine, 

 non-ciliated epithelium, and their relations to the gonaducts are best seen in the 

 series of sections, figs. 24, 25, and 26, drawn under a high power of the microscope. 

 The presence of specialised accessory organs in the shape of vesiculse seminales on 

 the gonaducts is, as far as I know, a unique feature among the Lamellibranchia, 

 though Pelseneer (12) makes mention of an accessory gland on the male duct of 

 Cuspidaria, but this gland is not described in his detailed account of the anatomy of 

 the genus. The numerous specimens of Heteropsammia and Heterocyathus, sent me 

 by Professor Herdman, were collected in February and March, and as the more 

 mature individuals of Jousseaumia inhabiting them are nearly all in the same sexual 

 condition, viz., in the protandric phase, it seems probable that in this genus there 

 is a seasonal alternation of male and female maturity. If this conjecture is right, 

 it is evident that the vesiculse seminales serve as reservoirs for the spermatozoa, which 

 are stored up until the ova are ripe and ready to be discharged from the gonaducts. 



Hermaphroditism, though it is not uncommon among the Lamellibranchia, is only 

 characteristic of a single sub-order, the Anatinacea. In all the hermaphrodite forms 

 protandry is the rule, as in Jousseaumia, and this is markedly the case in the Anatinacea, 

 as shown by Pelseneer (11). In this sub-order, the ovaries and testes are separate, 

 the ovaries being dorsally and the testes ventrally situated in the visceral mass. In 

 Pandora, Thracia, and Lyonsia, the oviducts and spermiducts open separately by 

 contiguous orifices on each side of the body, and the same is the case in Lyousiella, 

 but in this last genus the male and female apertures open very close together on a 

 small genital papilla (Pelseneer, 11). In Jousseaumia the conditions are different ; 

 the gonad is single and alternately male and female in function, and there is only a 

 single gonopore on each side. But its structure is interesting as indicating the 

 manner in which the separate ovaries and testes of the Anatinacea may have been 

 evolved. The dorsal and ventral diverticula of Jousseaumia correspond in position 

 with the ovaries and testes of the Anatinacea, and, as I have shown, they are 

 to a certain extent specialised, since the production of spermatozoa is nearly 

 exclusively confined to the dorsal diverticulum. If the two diverticula were to 

 become separate and acquire separate openings to the exterior and the function of 

 producing ova were confined to the one, and the function of producing spermatozoa 

 to the other, we should have a condition of things nearly identical with that of 

 the Anatinacea. It must be observed, however, that in the latter group the testes 

 are ventral, whereas in Jousseaumia the male diverticulum is dorsal, and at a later 

 stage both diverticula become female. 



