GLOCHIDIA — FACTOES UNDERLYING ENCYSTMENT 471 



not at all or only sluggishly to tactile stimuli." Hence, in way 

 of summary, these authors conclude that "the stimulus which 

 causes the contraction of the muscle and results in the attach- 

 ment to the host is, in the case of hookless glochidia, usually a 

 chemical one, but in that of the hooked forms it is mechanical." 



1. Hookless glochidia. In order to test general tactile sen- 

 sitivity, and particularly to aid in localizing the sensitive regions, 

 a human eyelash, attached to a holder, was used. Its fine, 

 tapering tip is relatively small, even in comparison to the tiny 

 glochidia, and with it the surface of a larva could be precisely 

 explored. An ordinary hair of the head, with a blunt, cut end, is 

 of too large caliber to be appropriate for this purpose. As will 

 appear later, however, a small stimulating point is not as effective^ 

 on the whole, as a larger, blunter surface. 



a. Lampsilis luteola. The external surface of the valves is 

 entirely insensitive. Even shells which are forced shut and held 

 in a closed position for a time recover fully when the pressure 

 is released. 



Touching the mantle appropriately, on the contrary, leads to 

 a very prompt and vigorous closing response. A narrow border 

 zone close to the shell rim is unresponsive (fig. 4). Of the re- 

 maining area, it is perfectly obvious that the half adjacent to the 

 hinge is far more sensitive than the rest. But the region of 

 greatest sensitivity lies on the ventral side of the adductor muscle; 

 an open active glochidium, touched properly here, will usually 

 close at the first application. In other, less responsive regions,, 

 several attempts may be necessary before the contraction occurs. 

 Closure is in most cases a sudden, vigorous, and uniform snap; 

 with some less active individuals the response is more deliberate. 

 Unless the tactile stimulus is apphed as a staccato jab, the hair 

 is caught by active glochidia. Such a prompt and even ex- 

 plosive response is essential to insure attachment under the 

 conditions encountered in nature; contact of a glochidium, for 

 example, with the gills of a fish, is but momentary, due to the 

 swiftness of the respiratory current. 



When glochidia, tactilly stimulated, close on a hair, they retain 

 a firm clasp upon it. As a rule, this probably continues until the 



