236 LESLIE B. AREY AND W. J. CROZIER 



This erosion continues with increasing severity until natural 

 death supervenes. By its ravages the canals containing the 

 aesthetes are laid bare and the sense organs therein contained 

 are destroyed. The empty canals are visible under a hand 

 lens. This phenomenon was noted by Plate ('01 a, p. 383) in 

 connection with forms possessing large extra-pigmental eyes. At 

 a length of 7 cm. the erosion is quite pronounced; the shell is 

 also frequently covered with adventitious organisms. We have 

 shown the organs of photoreception concerned in orientation to 

 be located in the tegmenta of the valves. There is thus a pro- 

 nounced correlation between the development of positive photo- 

 tropism toward daylight and the erosion of the shell, for animals 

 less than 7 cm. length are rarely found completely exposed to 

 sunlight. 



There are a number of instances in which animals alter the 

 sense of their phototropism depending upon the intensity of the 

 light. It is in general the rule that this alteration is one from a 

 photopositive condition in weak light to a photonegative con- 

 dition in strong light, rarely the reverse. The general rule holds 

 in the case of Chiton, and in other forms exhibiting general in- 

 tegumentary sensitivity to hght. The indifferent point dividing 

 the range of light intensities into a lower range, exciting positive 

 orientation, and a higher range, inducing negative phototropism, 

 shifts in Chiton toward the upper limit as age (and erosion) 

 advances. 



Hence it would appear that in this animal the excitation of a 

 smaller number of sense organs (in the eroded chitons) is equiv- 

 alent in stimulating power to the action of a lower intensity of 

 light upon a larger number of sense organs. This is further 

 evidence pointing to the constant action of light as the source of 

 the orienting stimulus. No case of this kind has previously been 

 examined. It will be interesting to make a further study of this 

 matter, in relation to temperature, for example. 



c. The method of orientation. In Chiton tuberculatus, es- 

 pecially in older, unevenly eroded animals, there are to be found 

 traces of a condition, more plainly evidenced in Ischnochiton 

 purpurascens, which is at first sight anomalous. Sometimes an 



