158 LESLIE B. AREY AND W. J. CROZIER 



V. Thermal excitation 215 



1. Behavior at different temperatures 213 



2. Local application of heat and cold 221 



3. Summary 223 



VL Photic excitation 224 



1. Effects of light 224 



a. Behavior in an illuminated field 224 



b. Results of partial illumination of the body 228- 



2. Differential sensitivity 230 



a. Shading 230 



b. Increased light intensity 232 



3 . Distribution and nature of photoreceptors 233 



a. For illumination 233 



b. For shading 234 



c. For increased illumination 235 



4. On the theory of phototropism 235 



a. The orienting stimulus 235 



b. The determination of positive and negative orientation. . . . 236 



c. The method of orientation 237 



5. Bionomic correlations involving photic behavior 23S 



VII. Chemical excitation 240 



1. Reactions to various substances 240 



2. The mode of excitation by solutions 246 



3. The chemoreceptors 250 



VIII. The nervous system and sense organs of Chiton 254 



I. INTRODUCTION^ 



Chitons of the species C. tuberculatus^ constitute, in appro- 

 priate situations, a conspicuous element in the shore fauna of the 

 Bermuda Islands. Their large size, their abundance, and the fact 

 that when mature the sexes are readily distinguishable by external 

 inspection (Crozier, '19) make these chitons favorable animals 



^ Many of the experimental observations used in this report were originally 

 obtained by L. B. A. during the summer of 1914, and were briefly reported at the 

 fifteenth meeting of the Zoological Society (Arey, '18 a); some phases of this 

 inquiry have since then been worked over independently by W. J. C, who has 

 added matter drawn from field studies and from further experimentation, and is 

 responsible for the actual writing of the paper. In 1914 our work was made 

 possible by grants from the Humboldt Fund of the Museum of Comparative 

 Zoology; we would express our appreciation of this support. 



3 Chiton squamosus L. was obtained by the Challenger expedition near Ber- 

 muda (Haddon, '86), and this is the only species listed by the early conchologists 

 for the Bermuda area (e.g., Jones, '88). Heilprin ('89, p. 176) speaks in addi- 

 tion of 'C. marmoratus Gmel.' and Verrill ('02, p. 497) makes incidental mention 

 of 'C. marmoreus.' Undoubtedly, the 'marmoreus' is intended to refer to the 



