482 LESLIE B. AREY 



CaCk: 



5N, All close promptly. 



IN. All close, but response begins only after one-quarter to one 

 minute and is slow and deliberate. 



N/4. No effect on one lot and practically none on another. 



Arranging the results on page 481, the order of anion stimulat- 

 ing efficiency is found to be. 



CI > Br > I. 



This coincides with the series found for Chiton (Arey and Crozier, 

 '19; a discussion of certain other conflicting results appears in this 

 paper as well). 



The order of kation stimulative efficiency for the alkali 

 chlorides is: 



K > Na > Li. 



The difference between Na and Li is slight, but Na appears some- 

 what more stimulating. This, however, reverses the order of 

 these kations established for a number of marine invertebrates, 

 as well as their action on various protoplasmic processes (Arey 

 and Crozier, '19). Including the divalent kations Mg and Ca, 

 the series becomes (compare Crozier and Arey, '19): 



K > Na > Li > Mg > Ca. 



An attempt was made to discover whether a chemical solution, 

 too weak in itself to evoke a reaction from glochidia, will lower 

 the tactile threshold by increasing the general protoplasmic 

 irritability. HCl and NaCl furnished no positive evidence. It 

 is possible that after treatment with weak KCl the tactile stimulus 

 need not be so accurately localized directly in the sensitive areas 

 as formerly; also with LiCl the response seemed perceptibly more 

 ready. At best the increase is but quantitative and of no practi- 

 cal significance. Hooked glochidia, activated to repeated snap- 

 ping by weak KCl (p. 483), were similarly tested, but it is not 

 certain that their tactile threshold was thereby lowered. 



Except at the highest concentrations, a distinct latent period 

 characteristically occurs between the application of a chemical 

 solution and the response; this is suggestive of a direct stimulation 

 of muscle without nerve mediation. 



