Chap. XIII.] GONADS OF MOLLUSCA. 50! 



serve as an organ for conveying the sperm to the 

 female. 



The cephalous Mollusc a, such as the mussel or 

 the oyster, retain the simple conditions of generative 

 glands, being, as are so many marine forms for which 

 the water serves, by its currents, as the carrier of the 

 products of the male to the eggs or egg receptacles of 

 the female, without any secondary sexual organs. In 

 general character and appearance also the male glands 

 closely resemble the female, and it is, no doubt, in 

 consequence of this that so many discussions have 

 arisen as to the monoecious or dioecious arrange- 

 ments of certain Lamellibranchs. 



As seen in ordinary cases, the glands are placed 

 on either side of the body, and each has a separate 

 orifice ; with a continuous outer wall, each gland is 

 broken up into a number of separate pouches, and 

 some of the epithelial cells on their inner face become 

 converted into ova or spermatozoa. Small at most 

 periods of the year, they become greatly enlarged at 

 the breeding seasons, when they occupy a large part of 

 the spaces in the body ; the ducts are ordinarily short, 

 and the ova, on escaping, make their way into pouches 

 in the gill chambers, where they are fertilised by the 

 spermatozoa which are brought in by the currents of 

 the water of respiration. (See page 221.) 



The elaborate investigations of Ryder seem to 

 have settled the problem of the sexual characters of 

 the oyster ; one difficulty in the determination arises 

 from the fact, that while the Portuguese and the 

 ordinary American oyster have the sexes separate, 

 the common edible oyster of Europe (Ostrea edulis) 

 has the sexes united. By a magnificent effort of 

 histological chemistry, Ryder has shown that if two 

 colouring matters (safranin-red and methyl-green) 

 are brought to bear on suitably-prepared sections of 

 the body of an edible oyster, the red-staining fluid 



