V GENITALIA OF FELECYPODA 1 45 



meiit of accessory glands, wliich are sometimes very large, and 

 whose precise pm-pose has in many cases not been satisfactorily 

 determined. 



Fclecypoda. — In the dioecious I'elecypoda, which form the 

 great majority, the reproductive system is simple, and closel}- 

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

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

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

 placed symmetrically at the sides or base of the visceral mass. 

 The oviduct is short, and the genital apertm'e 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 pussv'S through the afferent siphon. 



Hermaphrodite Peleeypoda are rare, the sexes being usuall}- 

 separate. The following are assured instances : PecUn glaher, 

 P. jacohaeus, F. maximus, Ostrea edulis, Cardium norvegicum, 

 Pisidium jjusillum , Cyclas cornea, Pandora rostrata, Asjjcrgillum 

 dicliotomiuii, 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 Aspergillus 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- 

 hrancJiiatcc (Cuspidaria, Poromya, 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 Pelecypoda 

 belong to forms decidedly specialised, while forms distinctly 

 })rimitive, such as Nueida, tiolenomya, Area, and Trigonia are all 

 dioecious. In Gasteropoda similarly, the least specialised forms 

 (the Amphineura, with the exception of the Neomeniidae, and 

 the Ehipidoglossa) 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 hermaphro- 

 ditism in the group is, to a certain extent, a sign or accompani- 

 ment of specialisation.^ 



Development of Fresh-water Bivalves. — The vast majority 



^ Pelscueer, Comptcs Rcndus, ex. p. 1081. 

 VOL. Ill L 



