174 BULLETIN OF THE BUREAU OF FISHERIES. 



have reached as advanced a stage of development as is attained by any unionid at the 

 time it leaves the fish. They closely resemble the young of Anodonta at the close of the 

 parasitic period, and upon examination have been found to possess the following struc- 

 tures: The anteriorandposterioradductor muscles; the ciliated foot; two gill buds on each 

 side; a completely diflerentiatad digestive tract, including mouth, esophagus, stomach 

 intestine, and anus; liver; the cerebral, pedal, and visceral ganglia ; otocysts ; the rudiments 

 of the kidneys, heart, and pericardium; while they also show a slight growth of the per- 

 manent shell around the margin of the shell of the glochidium (fig. 45 , pi. xii) . The larval 

 muscle has completely disappeared, although some of the mantle cells of the glochidium, 

 as well as the hooks of the shell, are still present. They crawl slowly on the bottom of the 

 dish by the characteristic jerking movements of the foot, after the manner of the young 

 of other species at a corresponding stage, although the valves of the shell gape more widely 

 apart and the foot is shorter and less extensible. We have not succeeded as yet in keep- 

 ing them alive for more than 10 days, but it is difficult in the case of any species to main- 

 tain young mussels of this age under laboratory conditions. 



One of these young mussels after removal from the cord is shown in figure 45, plate 

 XII, in which many of the organs of the adult or their rudiments are clearly indicated. 

 A comparison will show that it is essentially as advanced in its development as the young 

 of Anodonta when it is liberated from the fish (cf. Harms's figures, 1909, and also our fig. 

 47, pi. XII, of Symphynota costata). 



The conclusion is inevitable that we have here to do with a species which has no 

 parasitism in its life history, although the presence of hooks and other typical glochidial 

 structures would indicate that it has originated from ancestors which possessed the para- 

 sitic stage like other fresh-water mussels. The cord is undoubtedly to be interpreted as a 

 nutritive adaptation which arises in the marsupium during the early stages of gravidity, 

 since the young embryos are at first contained in an unformed viscid matrix and the cords 

 are a later product. 



The whole history of this exceptional species warrants a more detailed study, and 

 Miss Young is now engaged in such an investigation. When her work is completed we 

 hope that it may include the entire course of development, the method of formation of 

 the cords, and the rearing of the young mussels during a much longer period than has thus 

 far been possible. 



V. ATTEMPT TO REAR GLOCHIDIA IN CULTURE MEDIA. 



Since the relation of the glochidium to the fish is essentially a nutritive one, it 

 seemed to us that it should be possible to rear the larvse through the metamorphosis 

 artificially, provided a suitable nutritive medium could be found, and accordingly a 

 series of experiments, with this object in view, were undertaken at our suggestion by one 

 of our students, Mr. L. E. Thatcher. Although the result has thus far been entirely 

 negative, we have not despaired of ultimate success, and, since the experiments are to be 

 continued, a brief mention of the methods employed may be made in this place. 



