THE SENSORY RESPONSES OF CHITON 173 



example, more than 700 chitons, averaging 7 cm. length, were 

 fomid within a strip three-eighths of a mile long, their eroding 

 importance will be admitted to deserve examination, (A study 

 of this matter is being made.) 



By the time the tide has risen one-quarter, every chiton in an 

 intertidal group is found to have deposited a considerable mass 

 of fecal matter within the anal region of the mantle cavity. At 

 high tide they do not appear to defecate to any great extent. 

 There would seem, therefore, to be some rhythmic sequence of 

 feeding operations roughly coordinated with tidal events. This 

 might assist in the determination of a metabolic rhythm, which 

 might in turn receive expression in (tidal) rhythms of behavior. 



5. Respiration 



The respiration of most individuals of C. tuberculatus is also 

 subjected to the influences of tidal events. Under water, Chiton 

 obtains oxygen by means of a water current, passing inward 

 laterally along the girdle, through the gills, and escaping at the 

 anal end (fig. 7). Out of water, the gills are more or less con- 

 tracted against the dorsal wall of the ctenidial channel. Some 

 oxygenation may, however, occur out of water, since the gills 

 remain damp, and in nature the girdle is usually lifted from the 

 substrate to some slight extent, unless the creature be disturbed. 



The girdle is important for respiration, as the region in which 

 it is hfted from the substrate localizes the intake for the water 

 current. When completely submerged, this is commonly at the 

 anterior end. The incoming water then impinges upon the 

 dorsal surface of the proboscis ('palp')- Water is also taken in 

 at the sides of the body. The latter is exclusively the case when 

 the chiton is but partly submerged (i.e., with the anterior end 

 out of water). The girdle may be locally lifted in the form of 

 channels (fig. 7) or may be completely lifted. The water passes 

 up between the gills, and escapes under an elevation of the girdle 

 at the posterior end. This elevation is of somewhat variable 

 form, although always located between the posterior ends of the 

 right and left gill series. It is formed as a direct result of the 

 water current impinging on the inner ventral margin of the girdle. 



