vii MOLL USCA GENITAL ORGANS 225 



XX. Genital Organs. 



A. General. 



In treating of the genital organs of the Mollusca, we shall have to 

 consider (1) the gonads or germinal glands, those most important 

 organs, in which the reproductive cells (eggs and spermatozoa) are 

 formed ; (2) the duets through which these cells reach the exterior ; 

 and (3) the eopulatory organs. 



1. The gonads or germinal glands have already, in Section XVIII., 

 been recognised as completely or incompletely demarcated portions 

 of the secondary body cavity, and have been described in their 

 relation to the other divisions of that cavity. 



The gonads are paired and symmetrical in the Lamellibranchia and 

 Solenogastres, occurring in one pair. In all other Mollusca, only one 

 unpaired gonad is found. In very rare cases, such as that of some 

 hermaphrodite Lamellibranchs, which will be described later, there are 

 two pairs of gonads, one female and one male. 



The sexes are separate, among the Amphineura in the Chitonidce 

 and Chcetoderma, in many Lamellibranchs, in the Scaphopoda, among the 

 Gastropoda in the Prosobranchia (excepting a few Marseniadce and the 

 Falvata), and in all Cephalopoda. Hermaphroditism prevails among 

 the Amphineura in Proneomenia, Neomenia, and allied forms ; in many 

 Lamellibranchs, among the Gastropoda in the Pulmonata, Opisthobi'anchia, 

 and in the Prosobranchiate family of the Marseniadce. 



In hermaphrodite animals, it is the rule that the same gland, the 

 hermaphrodite gland, produces both eggs and spermatozoa, but in 

 exceptional cases there are in the same individual distinct male and 

 female gonads (testes and ovaries). This is the case, as already 

 mentioned, in certain bivalves, viz. the Anatinacea and the Septi- 

 branchia, which possess two testes and two ovaries. 



Position of the gonads. The long tubular hermaphrodite glands 

 of the Solenogastres, which are separated from one another by a median 

 septum, lie in the anterior prolongation of the pericardium, over the 

 intestine. In the Chitonidce,, the gonads are found in a similar 

 position, but are not in open communication with the pericardium. 

 In the Gastropoda they lie in the visceral dome, usually in its upper- 

 most part, between the lobes of the digestive gland. Where the 

 visceral dome has disappeared, the gonad with the intestine and the 

 digestive gland shift back into the primary body cavity above the 

 foot. The gonads in the Scaphopoda occupy a position similar to that 

 of the Gastropodan gonads, lying dorsally in the high visceral dome, 

 above the anus and the kidneys. The same is the case in the 

 Cephalopoda. The paired much-lobed genital glands of the Lamelli- 

 branchia lie in the typical position in the primary body cavity, above 



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