REPRODUCTION AND ARTIFICIAL PROPAGATION OF FRESH-WATER MUSSELS. 1 59 



responds principally to tactile stimulation, does not receive the stimulus to permanent 

 closure which is given by the presence of any foreign object inserted between the valves. 

 When a specimen does become attached to the surface of a fin, as is sometimes the case 

 (fig. 21 and 22, pi. ix, fig. 25 and 32, pi. x), it presumably gains its hold by catching 

 upon one of the ridges formed by the fin rays, for the hooks could hardly be used upon a 

 perfectly flat surface. Glochidia sometimes hold to the surface of a fin by a shred of 

 tissue, under which their hooks have caught, remaining there after all the neighboring 

 specimens are completely overgrown (fig 25, pi. x), only to be torn ofif later without 

 having caused any noticeable hypertrophy of the fin tissue. Figures 25 and 32, plate x, 

 show that glochidia may become overgrown either flat against the surface or upon edge, 

 and figure 24, plate ix, shows a young mussel leaving a surface attachment after a 

 parasitism of 74 days. 



The behavior and reactions of glochidia are of course significant in connection 

 with the actual attachment when once the glochidium is brought in contact with a 

 suitable part of the fish's body and receives the normal stimulus to close its valves. 

 The bringing of the glochidium against just that part of the fish is a matter of the chance 

 distribution in the water. Hence the distribution of the glochidia to the several fins 

 is determined solely by the number likely to be brought in contact with a given part 

 of the body. Those fins which brush against the bottom are always the more heavily 

 loaded and the numbers elsewhere depend upon the extent to which the glochidia are 

 kept suspended in the water. The importance of the mucus for the glochidia of Sym- 

 phynola and of the larval thread for those of ,4 nodonta and Unio in tangling the glochidia 

 into masses and drawing others against the fish when a single one has become attached 

 has probably been exaggerated, as explained in the section of this paper which deals with 

 the function of the larval thread. 



Optimum infections, as we shall term those which are close upon the limit of the 

 number of glochidia which a fish can safely bring through the metamorphosis, often 

 show the glochidia very closely set one after another, as in figures 22 and 23, plate 

 IX, and figure 25, plate x, and several hundred may be safely carried by a fish 3 or 4 

 inches in length. Prolonged exposure causes so heavy an infection of the margins 

 (fig. 19 and 20, pi. IX) that the fin tissue appears unable to overgrow the mass of 

 glochidia, and they then remain attached without overgrowth for a week or more. 



Figure 19, plate ix shows how on a part of the fin having no overcrowding normal 

 embedding occurred, while in the more crowded areas the glochidia were still uncovered 

 even seven days after infection. In the middle upper margin of this fin it would seem 

 that the overgrowth might well have taken place, for many cases like figure 25, plate x, 

 have been observed in which glochidia as closely set were properly embedded. The 

 failure of overgrowth in this region is probably due to the presence immediately after 

 infection of a greater number of glochidia many of which have since been detached. 

 In all cases of this kind a smaller number will finally become embedded than in an 

 infection where the fin has received more nearly the optimum load (fig. 21, 22, 23, pi. ix, 

 and fig. 25, pi. x), for the great majority drop ofif when the fin becomes so mutilated 



