326 FOSSIL OSTEEIU^ OF NORTH AMERICA. 



that absorption may be favored it is propelled through the intestinal 

 canal, which is peculiarly constructed so as to present as large an amount 

 of absorbent surface as possible. 



The expulsion of efi'ete matters in the oyster, beside faecal matters, is 

 accomplished by the respiratory system or gills and the organ of Bo- 

 janus. The first is concerned simply with the eliminaticn of the gase- 

 ous products which are a result of the vital actions of the animal, the 

 latter apparently with effete matters more or less nearly similar to the 

 urinary excretions of higher animals. To what exteut the liver may be 

 excretory in function it is not possible to state. 



The organ of Bojanus, a structure which appears to represent the 

 kidneys of the higher animals, is an inconspicuous organ in the oyster 

 as compared with its development in some other lamellibranchiates, 

 each as the fresh-water mussels. M. Hoek, of Leyden, and the writer 

 have been the first to definitely locate and describe this organ in the 

 oysters of Europe and America. It consists of a crescent-shaped mass 

 of tissue, indicated by the area hj in Fig. 1, lying on either side, just 

 below the insertions of the adductor muscle ^1/. In structure it is spongy 

 and canaliculated and very possibly glandular. It lies close against 

 the mantle on either side, and, in fact, extends somewhat into its sub- 

 stance. M. Hoek has traced the connection of this organ in the Euro- 

 pean oyster with the pericardiac cavity and the openings of the genera- 

 tive organs s, on either side of the ventral process of the body-mass/, 

 shown in Fig. 1. The common openings of the generative organs and 

 the organ of Bojanus will, therefore, probably have to be regarded as 

 urogenital outlets. 



The sexes are confined to distinct individuals in the American and 

 Portuguese oysters, but the common oyster of Europe is clearly herma- 

 phroditic, that is, the two sexes are more or less evidently combined in 

 the same individual. The mature ova of the American and Portuguese 

 species are of about the same size and measure about one five-hun- 

 dredth of an inch in diameter, while those of the common European 

 species are about one two-hundred-and-fiftieth of an inch in diameter. 

 In the latter we may find the male and female elements developed side 

 by side in the same ovarian follicles, but it also appears that there is a 

 preponderance either of eggs or of spermatozoa developed in many indi- 

 viduals, so that some are even practically unisexual. 



The reproductive organs are actively developed for only a compara- 

 tively short time, extending over a period of probably two or three 

 mouths. In the region of the Chesapeake the most important spawn- 

 ing period seems to extend over the months of June and July, but con- 

 siderable rij)e spawn may be found even much earlier and later than 

 this. Individual oysters may be occasionally found at almost any 

 season of the year with spawn more or less far advanced or quite ma- 

 ture. It is not uncommon to find the reproductive organs far advanced 

 in respect of functional development in the early spring months of 



