No. 633] SHORTER ARTICLES AND DISCUSSION 379 



intrnpigmental type are functional; they are also more reactive 

 than Tonka is, although in the latter extrapiu-mental eyes; are de- 

 veloped. Acanthochites, moreover, likewise with intrapiirni' mal 

 "eyes," is reactive to shading, as in the case of Chiton, while 

 Tonica is not. We are therefore unable to assign definite types 

 of irritability to the several forms of shell photoreceptors. 



The position taken by Nowikoff (1909), on morphological 

 grounds, that these organs are not related in genetic sequence, is 

 not inconsistent with such functional data as I possess. He re- 

 gards the intra- and extrapigmental eyes as being independent ly 

 derived from megalaesthete structures. It is possible to consider 

 that the megalaesthetes (or certain of them) are activated by 

 light, and that this kind of irritability is simply retained by eyes 

 of the extrapigmental variety, whereas eyes of the intrapigmen- 

 tal sort are in addition activated by shading. However, the local 

 activation of the girdle (of Chiton) by light and by shading 

 makes it necessary to believe that tegmental mircaesthetes (e.g., 

 of the girdle scales) may also be implicated in the photic irri- 

 tability of the shell-plates. As yet, experimental data for the 

 analysis of this problem is incomplete. The possible significance 

 of the number of shell-eyes present also needs to be investigated. 

 College of Medicine, 



157-260. 



1919. On the Use of the Foot in Some Molluscs. Ibid., Vol. 27, pp. 



359-366. 

 Crozier, W. J., and Arey, L. B. 



1918. On the Significance of the Reaction to Shading in Chiton. 



Amer. Jour. Physiol, Vol. 46, pp. 487-492. 



Heath, H. 



1899. The Development of Ischnochiton. Zool. Jahrb., Abt. Anat., Bd. 

 12, pp. 1-90. 



1904. The Larval Eye of Chitons. Proc. Acad. Nat. Sci., Philadelphia, 

 1904, pp. 257-259. 

 Moselev, H. N. 



