112 BULLETIN OF THE BUREAU OF FISHERIES. 



Approaching the subject unhampered by any preconception in favor of the older 

 views, but rather with the belief that the conclusions of Redi would also hold for the 

 bivalves, Leeuwenhoek records, in the 83d and 96th letters of his Arcana Naturae, the 

 presence of separate sexes in Anodonta and Unio, as evidenced by the presence of eggs 

 and spermatozoa in separate individuals, and gives some account of the development. 

 That he clearly apprehended the main course of events is evident if we read his descrip- 

 tion of eggs found floating free in the fluid obtained by puncturing the upper part of 

 the foot upon either side, of similar eggs in more advanced stages within the outer gills, 

 and of various stages in the formation of the glochidial shell. Finally, he observed the 

 snapping of the valves, now so well known as a sign of the last stages in this embryonic 

 development, and upon seeing the rotation of the embryo in the egg membrane he 

 concluded that it must be unattached. He further obsers^ed that the individuals, 

 when ready for their egg laying (passage of eggs from ovary to gills), placed themselves 

 in spots where the water was shallow and where they were in direct sunlight — a fact 

 which seems to have been confirmed by other observers of the European species (Schier- 

 holz, 1888, p. 8, Unio and Anodonta). Observing the general similarity between the 

 bivalved larva and the adult, he seems never to have doubted that the glochidia, as 

 they were subsequently called, were the young of the mussel in which they were found 

 and therefore that these mollusks were vdviparous, conclusions which so naturally fol- 

 lowed from all the facts that it is hard to see how convincing evidence could have been 

 manufactured for any other opinion. Upon removing these fully formed larvae and 

 setting them aside in dishes of clean water, with a view to observing their further 

 development, Leeuwenhoek met the stumbling block of all observers before the dis- 

 covery of the parasitism upon the fish was known, for the larvae lived but a short time, 

 soon becoming infested with a variety of animalcules, which he rightly concluded were 

 the immediate cause of their death. 



These conclusions of Leeuwenhoek, so nearly in accord with our present knowledge, 

 were not entirely accepted, because they did not become known to some investigators 

 even a century later and because there was still a considerable recrudescence of the 

 older conception of spontaneous generation. The opinion of Poupart (1706) that these 

 mussels were hermaphroditic gained ground and dominated during the eighteenth 

 century, although the larvae, when found in the outer gills, were always regarded as the 

 young of the mussel until, in 1797, Rathke offered an entirely different explanation and 

 erected for them a new genus, Glochidium, and a species, parasiticum. According to 

 this explanation, which came to be known as the Glochidium Theory, it was supposed 

 that these multitudinous larvae were not the young of the mussels at all, but parasites with 

 which they had become infested. Since Rathke's theory attracted considerable attention 

 at the time and was later supported ardently by Jacobson (1828), and since it has given 

 us the term glochidium, we may note in passing the evidence upon which it was based 

 as stated by its later champion. 



I. The form and organization of the little shells is entirely different from that of 

 the adult Unio and Anodonta. 



