1883.] on Oysters and the Oys'er Question. 343 



whence it escapes into the supra-branchial passages and chamber and 

 thence makes its way out by the cloacal chamber. The place of the 

 water thus swept out of the infra-branchial chamber is, of course, 

 made good by a corresponding flow between the pallial lobes into it. 

 Hence, while the oyster is alive, and in its proper element, a powerful 

 stream constantly sets in on the ventral and anterior side of the body 

 and pours out from the cloacal opening on the posterior side. The 

 direction of the stream is marked by the arrows in Fig. 2 (A). 



It is upon the proper maintenance of this current that the life of 

 the oyster depends. For these animals feed upon the microscopic 

 organisms, largely consisting of diatomaceous plants, which live in 

 the sea ; and as they possess no organs for seizing such food, they are 

 almost entirely dependent for their supply of nourishment upon the 

 indraught caused by the cilia on the gills, and especially upon those 

 which line the edges of the branchial plates and direct a portion of the 

 current towards the mouth. The anterior ends of each pair of hemi- 

 branchiae are attached between the two palps of the side to which they 

 belong. The applied surfaces of the palps, between which lies the 

 commencement of the mouth-cleft, are ridged and richly ciliated, so 

 that anything brought by the ciliary current of the gills is led 

 directly into the oral cavity. The cilia which line this eventually 

 drive it into the stomach. Thus the unimpeded action of the cilia of 

 the gills is essential to the nutrition of the oyster ; but it is not less 

 necessary to its respiration, to the carrying away of the waste products 

 of the renal and alimentary organs, and to the expulsion of the repro- 

 ductive products. For all these processes depend, either on the flow 

 of water through the lamina of the gills, or upon the current which 

 sets out from the supra-branchial and cloacal chambers. 



Hence the importance of tolerably clear water to oysters. If 

 turbid water, laden with coarse sediment, enters the infra-branchial 

 cavity, particles of mud, too large to be moved by the cilia, lodge 

 upon the gills, and, gradually obstructing the current, interfere with 

 the primary functions of feeding and breathing to such an extent as to 

 injure, or even to destroy the animal. 



It would be out of place here to give any account of the compli- 

 cated renal organs of the oyster, recently discovered and described by 

 Dr. Hoek. But it is necessary to notice the openings by which the 

 cavity common to them and the reproductive organs debouches. 

 These are the small slit-like apertures (Fig. 2 (A), ug.) situated one 

 on each side of the lower and front face of the trunk, which open 

 into the supra-branchial cavity. 



The nervous system of the oysters is more poorly developed than 

 that of any of their allies among the lamellibranchiate mollusks. 

 Only two out of the three pairs of nerve masses, or ganglia, which 

 these animals ordinarily possess have been clearly made out, while, 

 of these, the pair which is most likely to represent the sensorium of 

 higher animals is exceedingly small. Moreover, no organs of special 

 sense have been demonstrated. So that, if any reasoning from 



