V GENITALIA OF PELECVPODA 1 45 



scribed will be found to occur, particularly in the direction of 

 the development of accessory glands, which are sometimes very 

 large, and wliose precise purpose has in many cases not been 

 satisfactorily determined. 



Pelecypoda. — In the dioecious Pelecypoda, which form the 

 great majority, the reproductive system is simple, and closely 

 parallel in both sexes. It consists of a pair of gonads, which 

 are either ovaries or tests, and a pair of oviducts or sperm-ducts 

 which lead to a genital aperture. The gonads are usually placed 

 symmetrically at the sides oi- base of the visceral mass. The 

 oviduct is short, and the genital aperture is usually within the 

 branchial chamber, thus securing the fertilisation of the ova by 

 the spermatozoa, which are carried into the branchial chamber 

 with the water which passes through the afferent siphon. 



Hermaphrodite Pelecypoda are rare, the sexes being usually 

 separate. The following are assured instances : Pecten glahei\ 

 P, Jacohaeus, P. maximus^ Ostrea edulis, Cardium norvegicum^ 

 Pisidium pusilhim, Cyelas cornea^ Pandora rostrata, Aspergilliun 

 dichotomiim^ and perhaps Clavagella. The greater number of 

 these have only a single genital gland (gonad) on each side, 

 with a single efferent duct from each, but part of the gland is 

 male and part female, e.g. in the Pectens above mentioned. 

 Pandora and Aspergillum have two distinct glands, respectively 

 male and female, on each side, each of the two glands possessing 

 its separate duct, and the two ducts from each side eventually 

 opening near one another. It appears probable that the Septi- 

 hranchiata (^Cuspidaria, Poromga, Lyonsiella^ etc.) must also be 

 added to the number of hermaphrodite Pelecypoda which have 

 separate male and female glands. 



It is worthy of remark that all the hermaphrodite Pelecjq^oda 

 belong to forms decidedly specialised, while forms distinctly 

 pi-imitive, such as Nucula^ Soleyiomija., Area^ and Trigonia are all 

 dioecious. In Gasteropoda similarly, the least specialised forms 

 (the Am'phimura^ with the exception of the Neomeniidae, and 

 the Rhipidoglossa^ are dioecious. It is possible therefore that 

 in the ancestors of the Mollusca the separation of the sexes had 

 already become the normal type of things, and that herma- 

 phroditism in the group is, to a certain extent, a sign or accom- 

 paniment of specialisation.^ 



1 Pelseneer, Comptes Bendus, ex. p, 1081. 



VOL. Ill L 



