Xo.633] SHORTER ARTICLES AND DISCUSSION 



377 



the receptor cells proper, in the integument. It seemed profitable' 

 to attempt an analysis of the functional values of the several 

 types of photoreceptive elements to be found in different chitons. 

 Accordingly, in 1918 I made observations on the photic irritabil- 

 ity of representatives of several genera available at Bermuda. 

 Pending the collection of more information on this subject, which 

 is necessary for a full discussion of the problem here suggested, 

 I give briefly the net result of these observations. 



2. Ischnochiton purpurascens — found along the shores of 

 islands in Great Sound, and in bays on the south shore of Ber- 

 muda, usually in more or less exposed situations, but commonly 

 a little lower than the lowest reach of the tide (never between 

 tidal limits) — is quite sensitive to light. Individuals about 1 cm. 

 long were frequently obtained on bottles which had been on the 

 bottom long enough to acquire a film of algal growth ; the under 

 surfaces of such bottles, and of smooth stones, provided most of 

 my specimens. These animals were photonegative to light of 

 any intensity used — from very weak diffuse light to direct sun- 

 light. This species therefore resembles I. magdalenensis (Heath, 

 1899). It is said that among the Ischnochitonina there are no 

 shell eyes. However, Boreochiton, also of this family, never oc- 

 curs "far from the light" (Pelseneer, 1906, p. 50). 



/. purpurascens is an active creeper (Crozier, 1919). It 

 orients very quickly and precisely away from a source of illumi- 

 nation. At night its photic irritability seems decidedly en- 

 hanced, as I learned by comparing the rate of orientation of 

 single individuals to lamp light, in a dark room, at different times 

 during the twenty-four hours. (This would appear to be the 

 case with Chiton tiiln rcnlatns also — cf. Arey and Crozier, 1919.) 



No evidence was had that I. purpurascens is reactive to 

 changes of light intensity. 



3. Acanthochites spkulosus. Specimens about 14 mm. long 

 were found under stones, somewhat beneath low water level, in 

 Ely's Harbor and at Spanish Point. In these places the water of 

 the open ocean is less modified than within the sounds. The re- 

 quirements of Acanthochites seems in this respect more rigorous 

 than are those of the preceding species, for A. spiculosus was not 

 found well within Great Sound. As in the case of Ischnochiton, 

 the present species is decidedly photosensitive, and orients pre- 

 cisely away from the light. It moves faster away from a bright 

 light than from a weak one, and comes to rest in the shade. It 

 is strongly thigmotactic, tending to settle in the angles at the 



