REPRODUCTION AND ARTIFICIAL PROPAGATION OF FRESH-WATER MUSSELS. 1 53 



but in that of the hooked forms it is mechanical. The latter may be readily imitated 

 and glochidia of this type made to grasp firmly the point of a needle or the edge of a 

 piece of paper by simply touching them between the open valves. When once closed 

 in this manner they do not relax, but remain attached to the object until they die. 



The following statement made by Latter (op. cit., p. 56) has been frequently quoted, 

 especially in textbooks, but it has apparently never been verified or disproved. 



The Glochidia are evidently peculiarly sensitive to the odor (?) [sic] of fish The tail of a 

 recently killed Stickleback thrust into a watch glass containing Glochidia throws them all into the 

 wildest agitation for a few seconds; the valves are violently closed and again opened with astonishing 

 rapidity for 15-25 seconds, and the animals appear exhausted and lie placid with widely gaping shells, 

 unless they chance to have closed upon any object in the water (e. g., another Glochidium), in which 

 case the valves remain firmly closed. 



Although it is not stated that the tail which caused such a commotion among the 

 glochidia had been cut off from the fish, it is probable that such was the case. We have 

 repeatedly tested glochidia in the same manner both with fins and gills of different 

 fishes, and, providing that a bleeding surface is not brought in contact with the water 

 containing the glochidia, absolutely no response on the part of the latter takes place. 

 The result, however, is much as Latter describes if a little of the fish's blood gets into the 

 water in the neighborhood of the glochidia, except that our experience has shown that 

 after snapping for a few seconds they come to rest in permanent closure. It therefore 

 seems possible that the contractions seen by Latter were due to the introduction of some 

 blood with the tail of the fish, as otherwise agitation of the glochidia under similar con- 

 ditions has not been observed by us. 



Since the hooked and bookless glochidia, whose reactions to blood and to certain 

 salts we have studied, show important differences in their behavior, they are referred 

 to separately below 



REACTIONS OF HOOKLESS GLOCHIDIA. 



It was first observed that glochidia of the bookless type, in marked contrast with 

 the hooked forms, only occasionally exhibit spontaneous contractions and respond either 

 not at all or only sluggishly to tactile stimuli, and the question at once arose as to what 

 causes their closure when they become attached to fish. If the stimulus which brings 

 about a contraction of the adductor muscle in attachment is not a mechanical one, it 

 presumably is chemical in nature, but we were completely in the dark in the matter until 

 it was cleared up by the following experiments, the first of which were made with the 

 glochidia of Unio cotnplanatus at Woods Hole, Mass. 



When a small drop of blood of either the killifish, FundiUus diaphanus, or the white 

 perch, Morone americana, was placed over the glochidia contained in a small amount of 

 water in a watch glass, the effect was immediate and very striking. Every glochidium 

 was thrown into rapid and violent contractions, alternating with relaxations, the edges 

 of the valves either quite or nearly touching with each snap. Where the stimulus was 

 strongest — that is, immediately under the drop of blood — the glochidia exhibited two or 

 three strong contractions and then remained closed, but, proceeding outward to zones 



