474 LESLIE B. AREY 



deliberate. It is of undoubted benefit to the organism that this 

 condition should exist. A keener sensitivity near the hinge 

 insures a more liberal 'bite' and a firmer hold. This facilitates 

 encystment, which in fact may not proceed, in the hooked forms 

 at least, if only a shred of host tissue is caught. 



"Whether closure ensues only in case a hair cell is touched is 

 likewise difficult to determine. It is not easy to stimulate the 

 most sensitive regions and be sure that no part of the hair has 

 come in contact with the sensory cells, for the open glochidium 

 tends to rotate when thus touched. After prolonged attempts 

 to decide this point, I am led to conclude that one can stimulate 

 between the hair cells and yet obtain contraction. 



These results may be summarized in the statement that the 

 hair cells are sensitive to tactile stimulation, the inner pairs far 

 more so than the outer; yet it is believed that the glochidium 

 may be responsive when the hair cells are not directly touched. 

 The soft parts of the larva are delicate and the gentlest touch 

 causes a deformation of the tissue, as is directly observable; for 

 this reason it is not improbable that the application of a de- 

 forming pressure near a hair cell causes a traction which consti- 

 tutes an effective stimulus. It is conceivable that closure 

 normally follows a mechanical activation of the muscle, either 

 directly or by remote traction on the mantle; nevertheless, the 

 total experimental evidence does not favor such an interpretation. 



In a few cases when touching glochidia by a succession of 

 staccato prods (the hair being withdrawn before it could be 

 caught), the valves closed but partially — perhaps three-quar- 

 ters — and then slowly opened to the maximum. Possibly this is 

 to be interpreted as an optimal stimulation applied at the limit 

 of an area of indirect influence on the hair cells. Usually, 

 however, the response is 'all or none.' 



Spontaneous closure is sometimes seen among glochidia of a 

 particular lot. This may involve jerky contractive movements 

 which gradually bring the valves together, or it may be accom- 

 plished in a single movement by a quick or slow snap. In still 

 other cases there are spasmodic partial contractions, the valves 

 merely 'winking.' After closure, some sooner or later open again; 



