176 BULLETIN OF THE BUREAU OF FISHERIES. 



the foot reaches its limit of extension, the cilia stop abruptly and remain quiet while the 

 forward movement of the body is taking place, only to resume their activity when the 

 extension begins again. Figure 18, plate viii, furnishes an excellent illustration of 

 the various positions assumed as the young mussels crawl about in their twisting, jerking 

 movements, and also shows the extent to which the shell has grown beyond the limits of 

 the glochidial valves by the end of the first week of free life. 



In the great majority of forms, as appears from the work of other investigators and 

 our own observations, the mussel leaves the fish with only a very narrow margin of adult 

 shell protruding beyond the glochidial outline. The shape is still that of the glochidium, 

 although all other resemblances to this larval stage have disappeared. In the larva of 

 Symphynota costata this margin of the adult shell is so narrow, even after some days 

 upon the bottom (fig. 47, pi. xii), as not to protrude beyond the glochidial outline when 

 the young mussel is slightly contracted. Exceptions to this supposedly universal con- 

 dition have been obsers^ed by Coker and Surber (191 1) in the young of Plagiola dona- 

 ciformis and Lampsilis (Proptcra) IcBvissima — forms in which there is a considerable 

 growth of the definitive shell and presumably of the other organs during the parasitic 

 period. These cases are unique so far as known, but in view of the small number of 

 species which have been observed at all during this period of their existence other such 

 exceptions may be looked for. No data bearing upon the duration or other conditions 

 of the parasitic life are given in the paper in question, since the material studied was 

 from the gills of a fish which had been preserved after its infection under natural 

 conditions. 



These stages immediately following the parasitism and until the mussels are about 

 20 mm. in length are less known than any others. They have seldom been found by 

 collectors, and the reasons for this are made clear by the work of Isely (191 1), to which 

 we shall presently refer. Pfeiffer first observed and figured in 182 1 a small shell having 

 the glochidial outline still visible at its umbo, and other cases have been recorded, 

 notably by Schierholz (1888). Such specimens were taken from nature and not from 

 mussels artificially reared. Indeed, no one has yet succeeded in following individual 

 specimens for more than a few weeks beyond the beginning of life on the bottom. 

 Recently Harms (1907, 1908, and 1909) has obtained these stages, by rearing, more 

 extensively than his predecessors and has figured (1907a, p. 811) the young of Anodonta 

 with a very substantial increase in size at an age of six weeks after the parasitism, 

 beyond which they could not be reared because of their destruction by small Crustacea. 

 He concludes that the latter constitute a serious danger to the life of the young mussel. 



In our own work repeated attempts have been made to rear these stages to a size 

 which can be more easily handled, but without success. Specimens of Symphynota costata 

 (fig. 47, pi. xii) and of Anodonta cataracta have been kept alive in small dishes containing 

 green plants for a period of from one to two weeks after they had left the fish, and 

 Lampsilis ligamentina and subrostrata for a period of six weeks. Little or no growth 

 was observed after the first week. The two species of Lampsilis formed a conspicuous 

 border of new shell during the first few days of bottom life (fig. 18, pi. viii, and fig. 48, 



