"224 LESLIE B. AREY AND W. J. CROZIER 



species, account must be taken of the toughness of the tissue 

 concerned (the dehcacy of the respective cell surfaces). This 

 may explain why the delicate, internal, protected surface of the 

 oral siphon of Ascidia (Hecht, '18) exhibits a sensitivity to heat 

 and to cold superior to that known for many other animals. 



VI. PHOTIC EXCITATION 

 1. Effects of light 



a. Behavior in an illuminated field, a. Preliminary experiments. 

 Chitons collected more or less at random and without much at- 

 tention to size were tested in a qualitative way with reference 

 to their photic behavior. At the bottom of one end of a wooden 

 box, 29 cm. long by 23 cm. wide by 30 cm. deep, there was cut a 

 horizontal slot about 12 cm. long and 1 cm. high. This box was 

 coated on the inside with lampblack suspended in turpentine, 

 giving an approximately dead-black finish. A rectangular glass 

 jar containing sea-water to a depth of several centimeters was 

 placed inside the black box, within which it fitted closely. 

 Chitons were put in the glass jar, the box covered, light admitted 

 (or directed) through the slot, and the subsequent movements of 

 the animals determined. 



With diffuse sunlight twenty-one experiments upon twenty 

 individual chitons gave this result : 6 did not move at all during 

 the course of the test (lasting about one hour) ; 1 oriented a few 

 degrees away from the light, while 13 animals made definite 

 progress toward the light, irrespective of their original orien- 

 tation in relation to it. These chitons were probably all of 

 average size or larger. In some instances they were allowed to 

 become fixed to the bottom of the aquarium with their anterior 

 ends toward the light, in other cases they were placed with long 

 axis perpendicular to the light, in still others deliberately headed 

 away from it or quite at random. The nature of the result in 

 these experiments will be evident from the following record : 



