THE SENSORY RESPONSES OF CHITON 183 



proboscis (fig. 14), clearly marked off from the foot. The pe- 

 riphery of the proboscis is thin and very mobile, and reacts by 

 local contractions and bending movements when irritated. 

 During feeding the mouth opens in rhythmic fashion to permit 

 the extrusion of the subradular organ and lingual ribbon, as 

 Heath ('03) observed in Cryptochiton. The whole proboscis 

 may be retracted and temporarily covered by the forward ex- 

 tension of the anterior part of the foot; Heath ('99, p. 579; sep., 

 p. 4) noted that the proboscis of Ischnochiton magdallensis was 

 completely exposed in anuxials up to 4 mm. in length, but that 

 with further growth it became normally covered by the pig- 

 mented anterior part of the foot. The surface of the foot itself 

 is locally reactive, as shown by puckerings upon its surface and 

 along its margins due to contraction. The substance of the 

 foot may be swung, either as a whole or in any local part, to 

 one side or the other, and may also be considerably extended 

 as well as tightly contracted. The gills respond singly to local 

 stimulation by contracting in such a way as to be pulled dorsally 

 toward the wall of the gill channel. They may also, under 

 certain circumstances, exhibit synchronous movements. The 

 anal papilla is capable of movements of extension, retraction, 

 and sidewise bending. 



The neuromuscular mechanism of these movements is to a 

 large extent locally contained. If the head segment or the tail 

 segment be cut off, the tissues in the piece removed (head, sole 

 of foot) are reactive to touch; stimulation of the foot or of the 

 mantle causes the foot to be drawn toward the source of irri- 

 tation. The part of the chiton remaining after the amputation 

 also gives the customary responses, although the gill reactions 

 are usually weak. The 'shock' effect of such an operation is of 

 course severe, and is probably even greater when an animal is 

 bisected transversely. In this case both halves are reactive, 

 but the amplitude of the responses is much decreased, the gill 

 responses to touch being absent. The local 'reflex' character 

 of the regional movements is fully substantiated by experi- 

 ments to be described on subsequent pages. This condition is 

 reflected in the nervous architecture of Chiton, the central nerv- 



