THE SENSORY RESPONSES OF CHITON 205 



upon sand, upon mud, or upon a rock surface minutely studded 

 with sharp points; the negative reaction to surfaces of this 

 character is effective in determining the local habitat of Chiton, 

 since they do not occur upon muddy beaches nor upon sand, 

 nor do they at any time creep up upon the sharply pitted shore 

 rocks in the narrow zone which is covered at spring tides but other- 

 wise exposed to wind erosion. It may be noted in addition that 

 the young chitons, up to three years of age or more, and espe- 

 cially in very young stages, occur conspicuously upon smooth 

 stones. This is not altogether an accidental consequence of the 

 fact that the rock surfaces in the situations where their tropisms 

 force them to reside are frequently of a smooth character, since 

 the smallest specimens are found upon the under side of bottles 

 (of dark glass) below mean-tide level in company with Ischno- 

 chiton. 



4. Rheotropism 



It was noted that a number of chitons escaped from a col- 

 lecting pail, located at one end of a long aquarium table, and 

 that they tended to accumulate in the shallow gutter which 

 carried away the overflowing sea-water. A good number of 

 these animals moved down the gutter, with the current, even 

 though in so doing they traveled slightly down hill, against 

 their negative geotropism. Further tests, made in this gutter, 

 showed that, to currents of sufficient strength to produce any 

 effect, a majority of the animals were negatively rheotropic. 

 For a more refined test, chitons were completely submerged in a 

 trough of sea-water (a wooden fish-hatchery trough) through 

 which a current flowing at the rate of 5 to 10 cm. per second was 

 maintained. The rheotropic response was here less definite than 

 it appeared in the first observations, but was undoubtedly 

 negative. 



The rheotropism of Chiton is clearly a 'laboratory phenome- 

 non,' and may owe its appearance to the mechanical deformation 

 of the girdle by currents or to other tactile irritations, in either 

 case inducing negative orientation, or it may be traceable to a 

 deforming influence of the current upon the body as a whole, 



