240 LESLIE B. AREY AND W. J. CROZIER 



species from the standpoint we have developed in this paper 

 (Plate, '01 a, p. 509). Ischnochiton purpurascens, Acantho- 

 chites spieulosus, and a species of Tonicia we find to be photo- 

 negative to light of all intensities. The deep-sea solenogastres 

 may also be photosensitive, although the only observation known 

 to us on this point (Heath, '04a, p. 461) is not decisive. 



VII. CHEMICAL EXCITATION 



1. Reactio7is to various substances 



The surface of the soft parts of Chiton is sensitive to a wide 

 variety of chemical excitants. We are concerned, in the first 

 place, to discover something of the nature of the process of 

 chemical excitation by electrolytes. For this purpose experi- 

 ments are cited in which, unless otherwise stated, approximately 

 0.5 cc. of solution was locally applied to the surface of chiton, 

 tactile stimulation on the part of the stream being avoided. 

 The chitons were tested in air. After each response, the stimu- 

 lating fluid was washed away by a gentle stream of sea-water 

 before proceeding to any further tests. 



a. It was attempted to arrange certain salts in the relative 

 order of their stimulating power. The alkaline cations K, NH4, 

 Li, and Na, and Ca and Mg, were compared through the effects 

 of their chlorides, in 5/8 M solutions (made up in rain-water). 

 The mouth, the sole and the edge of the foot, the gills, and the 

 ventral surface of the girdle were stimulated in various indi- 

 viduals. The comparative efficiency of the different chlorides 

 was judged on the basis of their average effects upon all the 

 parts activated. Since it may be supposed that an effectual 

 subjective factor entered into these judgments, we will give rather 

 full notes of the experiments. 



KCl: Very strong stimulation in all regions, even on the ventral 

 surface of the girdle, which bends inward toward the source of excita- 

 tion. All the responses very sharp. 



NH4CI: Also gives pronounced contractive responses, but not so ex- 

 cessively strong as with KCl, nor of such long after duration. No 

 responses from the girdle. 



