172 LESLIE B. AREY AND W. J. CROZIER 



character of their sensory responses, which determine certain 

 features of their Hfe in these habitats, afford important evidence 

 to this effect. 



I^. Feeding 



All the chitons, probably, are vegetable feeders. They rasp 

 the thin coating of algae from the rocks by means of the radula 

 (H. Jordan, '13), which is long, armed with powerful black teeth, 

 and operated by a complex arrangement of muscles (Plate, '97). 

 The body musculature is also involved in feeding. The whole 

 body 'lurches' back and forth synchronously with the use of the 

 radula, the forward swing coinciding with the retraction of the 

 radula; the foot remains stationary. In rock crevices C. tuber- 

 culatus occurs frequently in groups, piled one animal upon 

 another. Investigation has shown that under these circum- 

 stances they may feed on one another's backs upon the algae 

 growing there. The radula removes not merely the algae, but 

 some of the rock surface as well. The chitons may be of some 

 slight geological importance in this way, and they may also be 

 in small part responsible for the destruction of the periostraca of 

 their associates and thus for the weathering of their shell plates. 



Most of their feeding seems to be done at high tide. It is when 

 covered with water that they move about most freely, although 

 in damp places they also move to some extent at low tide. 

 The great majority of the individuals are found well confined 

 within tidal limits. While exposed to the air as the water falls 

 they defecate copiously. The feces are discharged in the form 

 of tiny cream-colored, cigar-shaped masses, varying in length 

 with the length of the animal (fig. 1); in an animal 8 cm. long 

 the fecal masses are 3.1 mm. long and 1 mm. in greatest diameter. 

 The masses consist for the most part of minute granular bits of 

 sand, but contain also undigested plant remains and fatty globules. 

 When treated with acid, bubbles of CO2 appear; all but a shght 

 meshwork of algae fragments is dissolved. The mass of plant 

 fibers holds the fecal matter together in a pellet, which persists 

 for as much as twelve hours under water in nature. When it 

 is considered that, along the north shore of Long Island, for 



