114 BULLETIN OF THE BUREAU OF FISHERIES. 



the glochidium as having a very complex structure and possessing many of the organs 

 of the adult mussel, made a distinctly backward step; and his account of hearts, 

 stomachs, livers, intestines, and aortas, all highly developed and double in each indi- 

 vidual, reminds one of the description of elaborate systems of organs in the infusoria 

 as given by Ehrenberg in his monograph published during the same year. Pfeiffer (1821, 

 taf. II, fig. E) was the first to observe the minute outUne of the glochidium at the umbo 

 of a young shell — a fact which, had it become generally known, would have saved Jacob- 

 son his defense of the Glochidium Theory. There remained, however, the unexplained 

 gap between the glochidium and such a stage of the young mussel, and this was filled 

 only by Leydig's discovery of the parasitism. With the clue thus given, the stages by 

 which the glochidium becomes the miniature adult, during the course of its parasitism, 

 were studied by Braun (1878), Schmidt (1885), Schierholz (1878 and 1888), and more 

 recently by Harms (1907-19091. All of these investigators obtained their material 

 in great abundance by the artificial infection of fish with the glochidia, and in their 

 several accounts the structure of the glochidium and the organogeny of the common 

 European species will be found very completely given. 



The embryonic stages attracted new attention with the rise of cytological studies, 

 and the paper of Flemming (1875) was exhaustive for the period in which it was written, 

 although Lillie's more detailed and modern account (189,5) of the cell lineage and the 

 formation of the glochidium in Unto complanatus and Anodonta cataracia has rendered 

 Flemming's paper of historical interest only, and has apparently left undone nothing 

 of importance in a description of the early stages in these species. 



Further reference to the literature will be made as the several stages of the develop- 

 ment are discussed in the species we have followed. Since an excellent summary of the 

 literature, particularly that published since the paper by Carus (1832), may be found in 

 the work of Harms (1909), we omit further elaboration here. The report to the Paris 

 Academy (De Blainville, 1828) gives a good account of the Hterature for the earlier 

 period, and from this we have obtained a summary of the facts in such early papers as 

 have not been accessible. 



II. REPRODUCTION. 



The sexes are normally separate in the Unionidte, but in Anodonta imhecillis and in 

 a few other species of this genus the occurrence of hermaphroditism has been occasionally 

 recorded (cf. Sterki, 1898; Ortmann, i9i*). Although in the majority of the genera 

 of the UnionidiC the sexes are indistinguishable externally, in a few, notably in Lamp- 

 silis, the shell of the female differs from that of the male in its greater convexity in front 

 of the posterior ridge and in more or less well-marked differences in the posterior outline 

 of the shell. In such cases the males and females may be readily assorted without 

 recourse to an examination of the soft parts. 



At ovulation the eggs pass from the oviducts to the cloaca, and thence back into 

 the suprabranchial chambers, in which they are probably fertilized by spermatozoa 

 brought in by the respiratory current of water. From the suprabranchial chambers 

 they are conducted directly into those portions of the gills in which they are to remain. 



