GLOCHIDIA^ — FACTORS UNDERLYING ENCYSTMENT 473 



tor muscle, and these areas can then be found with moderate 

 precision. 



Whether or not a closing response follows only when the hair 

 cells are stimulated is not simple to determine. It is certain that 

 one can learn to evade those areas occupied by such sensory cells, 

 and, in most cases, not educe a response. Moreover, if the inner 

 mantle surface is explored by a series of equal staccato prods, it 

 is a common experience to elicit no response for several attempts, 

 when suddenly closure follows a stimulation appropriately placed. 

 This is apparently not due to cumulative stimulation, because 

 sensitive areas can be found where response follows a first 

 application almost invariably. Such regions are those which 

 presumably involve the touching of the inner sets of sensory cells. 



An attempt was made to obtain information on these points 

 by causing a glochidium to close on a hair, and then observing 

 where the tip of the imprisoned hair was placed. Great care 

 must be exercised, as the hair tends to swing about the clasped 

 region as a center, and then causes the tip to change its position. 

 In the majority of cases, the inner halves of the valves being 

 chiefly explored, the hair-tip after closure lay at the general 

 location of the anterior pair of inner sensory cells. Possibly the 

 structural relations are such that during stimulation the hair is 

 guided to this spot with greater surety than to the posterior 

 inner set. 



It has already been noted that the half of the mantle away 

 from the hinge is much less sensitive than the nearer half, and, 

 furthermore, that the outer sensory cells are the only ones that 

 can be satisfactorily seen in the living larva. It is a common 

 experience to touch or rub the outer cells vigorously several times 

 before eliciting a response. Due to the curvature and gaping 

 attitude of the valves, and the consequent tendency of the glo- 

 chidia to rotate, care must be exercised that in stimulating the outer 

 cells a more proximal part of the hair does not touch the inner 

 cells. However, it is apparently true that closure can be ob- 

 tained under these conditions when only the outer hair cells are 

 stimulated. It is perfectly certain that tactile sensitivity is not 

 acute in this region, and the response tends to be slower and more 



