350 DKECIOUS MOLLUSCA. 



consequently in a short century, fill our shallow lakes and 

 rivers, and raise to the sea's surface the banks on its hollow 

 bed, were it not for the numerous checks which oppose their 

 increase, and retain in equipoise the balance between them 

 and other entities and inorganic matter. In a single day, 

 says BuiFon, " a mass of oysters of several fathoms in thick- 

 ness is often raised ; the rocks to which they are attached 

 diminish considerably in a short time, and some banks are 

 entirely exhausted. The following year, howevei", fvirnishes 

 an equal quantity, and not the smallest diminution appears."* 

 Before I leave this section of my letter I would warn you 

 against the erroneous opinion of M. Rathke, which is that 

 the little shells found in the gills of the fresh-water mussels 

 are not their own fry but parasitical animals, which he ele- 

 vates into the rank of a genus under the name of Cyclidium. 

 M. Jacobson, a learned anatomist of Copenhagen, takes the 

 same view. He shows that the shape of the little shells 

 swarming in the gills is different from that of the presumed 

 mothers, for it approaches to the triangular, and in each 

 valve there is a small moveable and hooked denticle not 

 found in the adult, and a bundle of very irritable filaments 

 connected with the abdomen. Further, the fry in question 

 are of the same size and shape in all the different species 

 however various they may be in these respects ; and their 

 developement has no relation either to the season, or to the 

 age of the individual in which they are contained, while 

 their numbers seem to be enormous in proportion to the 

 number of the species of which they are imagined to be the 

 young. And Jacobson further deems it a too hard strain 

 upon our faith to call upon us to believe that organs so deli- 

 cately organized as the branchiae are, could be destined to 

 fulfil naturally the office of oviducts, and even of an uterus. 

 And yet it must even be so ; for in fact the whole progress 

 of the egg from the ovary to the gills, has been traced by 

 both competent and independent observers. Raspail con- 

 cludes his argument against their parasitism by asking — 

 " How can it be conceived that parasites should be contained 

 in a parcel like the eggs of the Mollusca, and ejected by the 

 animal itself ? " f 



II. DICECIOUS MOLLUSCA. 



These may be conveniently divided into two classes, the 

 first characterized by a perfect similarity between the male 



* Nat. Hist, trans, i. 213. 



t Cuvier's Analyse des Travaux de I'Acad. roy. des Sciences, ann. 1827. 

 p. 73. Edinb. New Phil. Journ. v. 405. 



