522 LECTURE XXI. 



The latest and best observations of naturalists and physiologists 

 on the sexual characters and generation of the Lamellibranchs have 

 established the correctness of Leuwenhoek's original conclusion* that 

 these mollusks are of distinct sexes, some individuals being male and 

 others female. In the small species of Anomia parasitic upon fuci 

 on the south coast of England, I have found the males and females 

 nearly equal in number, the males being distinguished by their 

 opaque white testis abounding in spermatozoa, the females by their 

 yellow or orange-coloured ovarium. 



Milne Edwards f has pointed out two substances, of different 

 colours, occupying the abdominal mass in the Pecten. The part to 

 which he restricts the term ovai'ium occupies the infei'ior and poste- 

 rior region of the mass ; and sends a duct which traverses a portion 

 of the different-coloured body situated above, and ascends to termi- 

 nate between the bases of the labial tentacles, the summit of the ab- 

 domen, and the anterior ends of the gills. To the opaque whitish 

 mass occupying the upper and larger half of the abdominal mass he 

 gives the name of testis : it is composed of small vesicles grouped in 

 bunches ; its duct is continued into the foot, and terminates by two 

 small orifices opening upon the inferior fissure of that organ4 These 

 Siebold suspects may be the orifices of the secreting glands of the 

 byssus, and Edwards has perhaps mistaken the gland and duct of the 

 byssus, with part of the undeveloped ovary, for the testis. He did 

 not apply the only true test of the nature of the uncoloured part, 

 viz. the microscopical examination of its contents. 



Krohn § has dissected a so proved hermaphrodite Clavagella : he 

 found the testis beneath the liver, while the ovary surrounded it and 

 the stomach : this appears to be an exceptional condition in the class, 

 though perhaps constant in that peculiarly domiciled genus. 



In the male Lamellibranchs the testes are double, and have a 

 somewhat more circumscribed form than the ovaria, but sometimes 

 appear to be blended together at the median line. In the oyster 

 they are situated on each side of the liver, and extend in the form of 

 a triangular process between the adductor muscle and the gills. The 

 testes extend at the breeding season in certain genera, as Anomia, 

 Hiatella, Mytihis, and 31odiola, into the substance of the lobes of the 

 mantle ; but in those bivalves which have a large foot, the testes are 

 confined to the base of that organ. The ultimate texture of the testes 

 is a congeries of vesicles containing a milky fluid, which seems to 



* XXXIX. p. 16. t CCCXXV. pi. 10. fig. 1. p. 322. 



X XXIV. p. 2«9. § CCCXXIII. p. 52. 



