REPRODUCTION AND ARTIFICIAL PROPAGATION OF FRESH-WATER MUSSELS. 1 I I 



possible data bearing upon their distribution, their habits, and the physical and bio- 

 logical factors of their environment, as well as information concerning the industries 

 which depend upon the mussel. Surveys of this character have now been carried out 

 on the Mississippi River and nearly all of its more important tributaries from Minnesota 

 to Tennessee, and as a result of these investigations an enormous amount of material 

 and information has been collected which, when examined and analyzed, will not only 

 have the greatest economic value, but will constitute one of the most important eco- 

 logical studies ever made on any group of animals. 



I. HISTORICAL. 



As has long been known, the Unionidse carry their young in the gills, which function 

 as brood pouches until the completion of the embryonic development. At the close of 

 this period the larva or so-called glochidium is fully formed and escapes from the egg 

 membrane while still within the gill. In some species the discharge of the glochidia 

 takes place at once, while in others they remain in the brood pouches for several months 

 without further change before being set free into the water. 



The glochidium, long thought to be a parasite infesting the gills and known as 

 Glochidium parasiticiim, was proved by Carus in 1832 to be the lar\a of the mussel 

 itself, although many years earlier Leeuwenhoek had given it the same correct inter- 

 pretation. In i866Leydig made the important discovery that the glochidium, after leav- 

 ing the parent, completes its development as a parasite on fishes. 



The earliest observations of importance in the development of our knowledge 

 concerning reproduction in the Unionidae are those of Leeuwenhoek, made about 1695 " 

 and recorded in the Arcana Naturae. During the two preceding centuries the belief had 

 gained ground that the mollusks had sexes like the higher animals, and this no doubt 

 helped to arouse a certain skepticism regarding the existence of any process of spon- 

 taneous generation among the representatives of this phylum. The obser\'ations of 

 Redi (1668), in disproval of spontaneous generation in insects, furnished collateral 

 evidence and appear to have been the direct incentive for Leeuwenhoek's examination 

 of the reproductive processes in certain mollusks, among others the fresh-water mussels, 

 and the discovery by Leeuwenhoek of eggs and sperm in these mollusks convinced him 

 that their reproduction must be effected by such means rather than by spontaneous 

 generation. 



It is surprising to find how accurate were Leeuwenhoek's conclusions regarding the 

 general course of the development as far as the larval stage, later known as the glochid- 

 ium, and a survey of the subsequent literature shows that not until the work of Carus, 

 in 1832, were there published conclusions more in accord with the facts as now known, 

 nor a better summary of what we now term the embryonic period. The correctness of 

 these early observations, so far as they went, and of the conclusions drawn from them 

 have not been sufficiently recognized in most accounts of the literature, and for this 

 reason an explicit statement of their important features is desirable. 



a The date of the publication referred to in the literature list is somewhat later. 173a. 



