476 LESLIE B. ABEY 



Exploration with a hair proves that they respond readily when 

 appropriate regions are stimulated, the most sensitive spot being 

 about one-third distant from the flaring ventral border. Stim- 

 ulation near the hinge educes responses of a slower, deliberate 

 nature. 



The narrowness of the valves makes the tactile stimulation 

 with a hair less easy to apply than in many forms. To a thin 

 shaving of cork, shaped like a gill filament, or to an actual gill 

 filament, t'hey respond more avidly. These objects fit better 

 between the valves and come in contact with a broader surface 

 than does the tip of a hair. 



3. Hooked glochidia. The mature glochidia of Symphynota 

 complanata and Arcidens confragosus are promptly reactive to a 

 hair (figs. 1 and 2), particularly a blunt one, but like other forms 

 they respond even more readily to a soft body offering greater 

 surface. The movement of closure is, however, rather deliberate, 

 and once shut the closure is permanent. In my experience 

 neither are they as delicately sensitive, nor is the response as 

 vigorous, as in some of the hookless forms. Why Lefevre and 

 Curtis ('12, p. 155) (working largely with Symphynota) should 

 draw the following conclusion, is not evident: ''Hooked glochidia, 

 in striking contrast with the behavior of the hookless forms, 

 respond very actively to tactile stimuli " 



Anodonta corpulenta is extremely sensitive to delicate tactile 

 stimulation — possibly more so than any other form observed 

 by me. To a hair it responds by a sudden, quick closure from 

 its widely open resting position. It may then promptly relax, 

 and, if the hair is withdrawn, give several snapping movements. 



When these hooked glochidia become attached to a suitable 

 object, such as a cork shaving, the closure is permanent. 



It was attemped to determine whether the threshold to tactile 

 stimulation is altered just before or after a spontaneous con- 

 traction, but no appreciable changes were detected. 



