THE SENSORY RESPONSES OF CHITON 197 



central 'rib' of each filament is b}^ far the most sensitive portion; 

 it is possible to pass a fine needle between the gill filaments with- 

 out getting any appreciable response, although the filaments 

 could be seen to move. 



By employing a finely twisted bit of tissue-paper or a minute 

 air bubble, it was possible to obtain the contraction of a single 

 filament when delicately touched, although generally at least the 

 two immediately adjacent ones were involved. 



This result, taken together with the low tactile irritability of 

 the lateral borders of the gill filament, shows that the antero- 

 posterior spreading of the gill response when the mantle is touched 

 is a nervous matter, and is not merely the result of one contract- 

 ing filament mechanically involving its neighbors. This is also 

 confirmed by tests upon animals having the pallial nerve strands 

 sectioned at various levels. The operation produces no serious 

 disturbance. The response of the homolateral gill series to fairly 

 severe tactile irritation at one spot does not spread past the 

 level of the cut 'nerve,' but ceases abruptly at this point (al- 

 though the gills themselves have been quite undamaged). 



The tactile sensitivity of the ctenidia is important for the 

 efficiency of respiration. Foreign particles (e.g., sand) drawn 

 into the gill channel by the respiratory current strike against the 

 ventral surface of the gills, inducing a sudden local depression of 

 the girdle, which squirts water out from under the girdle, remov- 

 ing the foreign object. 



h. The surface of the head, the 'palp,' and the region of the 

 mouth, are very sensitive to touch, the reactions produced being, 

 however, merely local contractions; to more vigorous excitation, 

 the animal responds by rolhng up. The free margin of the head 

 region is especially sensitive. 



i. The edge of the foot, when touched in the region of the gills, 

 induces responses such as those already described for the mantle 

 lining of the ctenidial chamber. It is very sensitive. The pos- 

 terior end of the foot yields responses like those of this region 

 of the mantle. 



j. The sole of the foot reacts by puckering away from a source of 

 tactile irritation, such as a blunt glass needle. To larger areas it 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 29, NO. 2 



