288 



LECTURE XXI. 



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

 equal in number, the males being distinguished by their opake white 

 testis abounding in spermatozoa, the females by their yellow or orange- 

 coloured ovarium. There appears indeed to be only one observed 

 exception to the dioecious condition, namely in the Cyclas., in which 

 Wagner found, in addition to the ovaria, an isolated pair of testes. 

 In the males the testes are double and have a somewhat more cir- 

 cumscribed 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 be- 

 tween the adductor muscle and the gills. The testes extend at the 

 breeding season in certain genera, as Mytilus and Modiola, into the 

 substance of the lobes of the mantle ; but in those bivalves Avhich 

 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 con- 

 taining a milky fluid, which seems to consist almost wholly of sper- 

 matozoa in the breeding season. The vasa deferentia are short and 

 wide, and they open behind the mouth in the oyster, and terminate 

 upon papillae at the posterior part of the foot in most dimyary mol- 

 Insca, as the Cardium, Pholas, Venus, &c. 



The ovaria have a similar form and position in the female bivalves, 

 but are usually more extensively ramified. At all seasons of the year 

 some ova may be discerned in the ovarian cells, characterised by the 

 germinal vesicle and spot. Towards the breeding season they are 

 developed in immense numbers ; and the addition of the coloured 

 vitellus to the essential part of the ovum gives the characteristic 

 colour to the ovaria. They are generally distended with the ova 

 in the winter months. The fertilising filaments retain their in- 

 fluence after being discharged from the males ; are drawn in with the 

 respiratory currents, and at the breeding season the ovaries and 

 oviducts contain a milky fluid abounding 

 with the moving filaments. The ova then 

 escape by the short oviducts, which ter- 

 minate in positions analogous to those of 

 the vasa deferentia. They are conveved 

 along the basal margin of the internal 

 branchioe, enveloped in mucus, from the 

 oviducts to the posterior terminations of 

 the inter-branchial space, where they en- 

 ter the canal which traverses the base of 

 the external gill, and pass into the recep- 

 tacles formed by the interspaces of the 

 transverse lamellae, connecting the outer 



