80 BULLETIN OF THE BUREAU OF FISHERIES. 



the circumstances of their removal from the river rather than the change in tempera- 

 ture may have caused them to become detached. An examination in March of the 

 following spring revealed the byssus present in most of the individuals, and it was present 

 until June 10, after which date it could no longer be found. The disappearance at this 

 time near the middle of the season's growth requires some explanation. It comes at 

 the beginning of the period of most rapid growth, which is, perhaps, a decided physio- 

 logical change, although very gradual, coming as it does after two months of spring 

 growth. The observations to be recounted of a byssus in adult mussels would lead 

 one to expect the persistence of the byssus under favorable conditions. On August 

 14, 1914, the author found an adult Plagiola donacijormis on a byssus, and later E. A. 

 Martin showed the author a still larger individual. The byssi in these cases were strong 

 enough to support the weight of the mussels. In this species {Plagiola donacijormis), 

 then, we find the byssus habit not confined to the juvenile stage. 



The development of the reproductive glands in fresh-water mussels was clearly 

 made out by Herbers (1913) in Anodontas and Unios. He was able to distinguish 

 early stages of the glands in Anodontas of 5.7 millimeters length. The maturity of these 

 organs would mark the adult stage. In collecting various species of mussels in the 

 field one occasionally discovers remarkably small individuals breeding. As these are 

 so uncommon they are undoubtedly examples of precocity and exceptional. 



The author has not found gravid individuals of Lampsilis luteola under what was 

 apparently the third year. In the cultures here described sexual differentiation in 

 secondary characters appeared the second summer. Modifications of the gills to form 

 the marsupia appeared in the female, together with the corresponding fullness of the 

 shell over that organ. The males were marked by the more pointed posterior portion 

 of the shell. In the middle of August of the third summer the first gravid mussels were 

 found. This, the first observed date of breeding, was 2 years, 2 months, and 24 days 

 from the date of implantation of the glochidium. All females as far as examined were 

 found to be gravid, which indicates that breeding is general at this age. The glochidia 

 were mature in some individuals on August 14 and near maturity in others, which from 

 the date of last observation would fix the time of ovulation as July. 



Mature glochidia from these mussels were taken and an implantation obtained on 

 a number of fish. The first free juveniles after metamorphosis were obtained in 10 days, 

 others remained as late as the 18th day, a rather long period of shedding. The juveniles 

 obtained represent the second generation of mussels, but the life cycle was completed 

 when glochidia were obtained, as that was the stage with which the experiment began. 



HABITS AND HABITAT OF JUVENILE MUSSELS. 



The juvenile or postparasitic period begins with the release of the young mussels 

 from encystment on the host. Because of the small size of mussels at this stage infor- 

 mation regarding their habits and environment must depend largely upon studies under 

 conditions of control or experiment. Obviously, it is entirely impracticable to count 

 on finding them thus early in nature. The watching of the process of separation from 

 the host has been found practicable only by making cuttings of infected gills from living 

 fish and by examinations under the microscope. The first sign of the change is a 

 repeated opening and closing of the shells. This is followed by extension of the foot, 



