152 BULLETIN OF THE BUREAU OF FISHERIES. 



two and the glochidia then become entirely free from one another. When taken from 

 the parent gill the glochidia of Symphynola are entangled in a ropy mucu?, and this acts 

 in a manner similar to the threads of Anodonta, but it is usually dissolved after a few 

 hours in the water. In the ripe glochidium of U. complanatus the threads are extruded 

 immediately after the glochidia are removed from the parent and placed in water, and, 

 according to Harms (1907b, p. 819), the minute glochidia of Margaritana tnargaritijera 

 extrude their threads while still within the egg capsule. 



When this extrusion has taken place in Unio covipla^mtus the glochidia and broken 

 egg membranes become united into globular masses from which it is difficult to separate 

 individual specimens, and from observing such glochidia in contact with the fish we 

 are forced to conclude that they are not so likely to become attached to the gills or fins 

 as they are later, when they have been separated by the disintegration of the threads. 

 The glochidia of Lampsilis, which when fully ripe fall apart into masses of entirely 

 unconnected individuals, appear much better able to attach to the gills of fishes imme- 

 diately after their discharge from the parent. We believe, therefore, that the thread is 

 something to be gotten rid of rather than an organ of great importance in the attach- 

 ment to fish, and this is in agreement with Lillie's interpretation of this organ as an 

 excretory product. It is possible that some homologue of the thread exists in these 

 threadless glochidia, and a comparative study of the pre-glochidial stages might yield 

 material for interesting comparisons. 



BEHAVIOR AND REACTIONS OF GLOCHIDIA. 



At the time of spawning the glochidia, already freed from the egg membranes, and 

 usually held together in slimy strings, are discharged at irregular intervals. Being 

 heavier than water, they sink rapidly to the bottom, coming to rest wit4i the outer sur- 

 face of the shell directed downward and the valves gaping widely apart. The belief was 

 formerly general that they "swim" about by rapidly opening and closing the valves, 

 after the manner of Pecten, and, in spite of frequent denials by Schierholz (188S), Latter 

 (1891), and others, the same statement is still occasionally encountered. In the recent 

 volume on Mollusca in the Treatise on Zoology, edited by Lankester, this inexcusable 

 error is repeated. "The glochidia," we are again informed, "swim actively by clapping 

 together the valves of the shell" (p. 250). ' They are, on the contrary, as is now well 

 known, entirely incapable of locomotion and remain in the spot where they happen to 

 fall, although it is true that they may exhibit from time to time spasmodic contractions 

 of the adductor muscle, which cause the valves to snap or wink, each contraction being 

 immediately followed by relaxation and opening of the shell. These movements of the 

 valves, however, are never so vigorous as to cause the glochidium to move from place 

 to place in the water. 



The glochidia remain in this helpless situation until they die, unless they happen to 

 come in contact with the host on which they pass through the post-embryonic develop- 

 ment as parasites. The stimulus which causes the contraction of the muscle and results 

 in attachment to the host is, in the case of hookless glochidia, usually a chemical one. 



