LAMELLIBRANCHIATA. 523 



consist almost wholly of spermatozoa 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 moUusks, as Cardium, Pholas, 

 Verms, &c. 



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

 but are usually more extensively ramified. The short oviduct and 

 sperm-duct are both strongly ciliated, and open on each side at the 

 base of the foot or visceral mass, by a narrow fissure with tumid 

 borders, either very close to the opening of the kidneys into the 

 mantle-cavity or in the urinary sac itself. 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. It is by virtue of the currents produced by the 

 action of the vibratile cilia of the mantle and gills that impregnation 

 is effected in the separate sexes of bivalves, and especially of those 

 that, like the oyster, are fettered, male as well as female, to the rocks ; 

 just as the pollen of the rooted male of the dioecious palm is wafted 

 by currents of air to the moist stigma of the equally fixed and rooted 

 female tree. The fertilising filaments retain their influence 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 terminate in positions analogous 

 to those of the vasa deferentia. 



The ova have usually a spherical, rarely a pyriform or elliptical, 

 figure : the yolk {Jig. 194, a b), when mature, varies from a yellow 

 to a reddish hue ; it is inclosed by a smooth vitelline membrane, and 

 this by a thin chorion. The (often overlooked) germinal vesicle (a) 

 has usually two co-attached nuclei. The small amount of albumen 

 (c) which lies between the yolk -membrane and the chorion becomes 

 more conspicuous after impregnation. 



The ova do not escape when excluded from the oviduct, because 

 during that act the shell is kept forcibly closed ; and so they slip 

 back into the gill-channels. In the Naiadm the two external gills are 

 developed into uteri or marsupial pouches for the ova, which cleave 

 loosely together in the compartments of those gills ; and in the fresh- 

 water genus Unio they are excluded by the anal fissure, near to 

 which they form oval discoid masses, in which may be recognized the 



