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THE AMERICAN NATURALIST [Vol. LIV 



NOTE ON THE PHOTIC SENSITIVITY OF THE 

 CHITONS 1 



1. The remarkable sensory organs discovered byMosely (1885) 

 in the tegmentum of the shell-valves of certain chitons are struc- 

 turally of such a nature that in their most highly developed 

 forms they were from the first recognized to be "eyes." Prac- 

 tically nothing has been made known as to the functional values 

 of these organs, which in different genera occur in a great diver- 

 sity of form, number, and arrangement. It has been shown, 

 however, that the tegmental aesthetes of Chiton tuberculatus are 

 indeed photosensitive (Arey and Crozier, 1919). But the shell- 

 eyes are in this genus generally represented by structures of an 

 intermediate degree of complexity. The "eyes" are supposed to 

 have been derived from large, relatively undifferentiated shell 

 receptors (macraesthetes), and seem to reach their highest de- 

 velopment in those species of Schizochiton and Tonka which pos- 

 sess large complex eyes, each surrounded by a pigment cup (cf. 

 Plate, 1899; Nowikoff, 1907, 1909) ; in Chiton (at least in some 

 species of this genus) the eyes are "intrapigmental," pigment 

 being contained within the receptor cells, whereas with the 



