378 



THE AMEBIC AN NATURALIST [Vol. LI V 



corners of an aquarium, and once in such a situation is difficult 

 to move by light. Negative geotropism is also fairly well pro- 

 nounced. 



If the intensity of light falling on an Acanthochites be sud- 

 denly increased, the girdle is depressed into contact with the sub- 

 stratum. Local illumination confined to the girdle leads to a 

 local response of the same character. The shell plates seem not 

 to be sensitive in this respect. 



As in the case of most Chitons (Sampson, 1895; Crozier, 

 1919; Arey and Crozier, 1919), the body may be strongly curved 

 to one side, the animal pivoting in a circle of short radius. Photic 

 orientation is often accomplished in this way. The "pivoting" 

 of Acanthochites ceases instantly when the creature is shaded; 

 orientation is resumed when the light is increased. Since the 

 girdle does not respond to shading of this part alone, I am led to 

 believe that the shell plates are probably responsible for this 

 type of reaction (as with Chiton; Arey and Crozier, 1919). 



4. As elsewhere described (Arey and Crozier, 1919), the shell- 

 plates of Chiton tuberculatus contain receptors activated by light 

 of constant intensity and by shading. 



5. An unidentified species of Tonica, about 6 mm. long, com- 

 monly obtained in company with Ischnochiton purpurascois, 

 was found not to be reactive to shading, nor to increase of illu- 

 mination; but, like the latter, was decidedly photonegative. 

 This form is not so reactive to light as Isch nochiton, however. 



6. Plate (1901) considered it possible that the order of evolu- 

 tion of the shell eyes of Chitons was from megakesthetes to intra- 

 pigmental eyes to extrapigmental eyes. In the present series of 

 species, this order would be represented by Ischnochiton, Chit mi, 

 and Tonica, in respective sequence. The shell eyes are of course 

 not the only photoreceptors in these animals; for the girdle 

 the ventral surfaces of the body (Arey and Crozier, 1919), and, 

 possibly, the bilateral larval ocelli (Heath, 1904) are functional 

 in this respect. But the experiments recorded in this paper show 

 that funetions of a certain diversity are served by the tegmental 

 photoreceptors of the several species. Little can definitely be 

 said, however, regarding the correlation of structural features 

 with functional performance. It is noteworthy that members of 

 the Ischnochitonina? — a group characterized by the absence of 

 shell "eyes" (i.e., with megakesthetes and micrfesthetes only) — 

 are quite as reactive to photic irritation as are members of Chiton 

 proper, where, so long as the tegmentum is uneroded, eyes of the 



