THE SENSORY RESPONSES OF CHITON 211 



sition is nevertheless defensible that they are tactile receptors 

 and that the responses to which they lead are not aspects merely 

 of the activity of generalized (' universal') sense organs. The 

 positive response of the foot to contact with a sufficiently large 

 area is significant upon this point, since this type of reaction is 

 never obtained upon chemical stimulation of the foot. 



The tubercles and 'hairs' upon the girdle of most chitons are 

 characteristically embedded in the cuticula of the integument and 

 connected at their proximal ends with secretory and probably 

 also with sensory cells. By means of movements of these stiff 

 projections, relatively slight mechanical disturbances may be 

 transmitted through the thick cuticula. We have shown that 

 on the shell surfaces of Ischnochiton the projecting 'hairs' are 

 open to tactile activation. In Acanthochites spiculosus the 

 girdle is regularly beset with circularly arranged groups of long 

 spicules. We find these spicules to exhibit an interesting reac- 

 tion. If one of them be touched, ever so hghtly, the rest of the 

 spicules in that group react also, the whole bundle being spread 

 widely apart. Usually all the bundles on one side respond if a 

 single spicule is disturbed. As the spicules are 5 mm. long, in 

 an animal 15 mm. long, a slight touch exerts a considerable lever- 

 age upon the base of the spicule. At rest the spicules are directed 

 posteriorly, but when disturbed the axis of each bundle is directed 

 more or less perpendicularly to the surface of the girdle, the 

 spicules themselves being spread wide apart; this results in a 

 disposition of their sharp points which may have protective 

 value. 



The 'scales' on the girdle of Chiton tuberculatus are homol- 

 ogous to the spicules of Acanthochites (Plate, '01 a), and their 

 functional significance for tactile reception is of a similar kind. 

 At the outer margin of the girdle are short, stiff 'hairs,' directed 

 normally to the periphery, which also function in this way^ 

 (Plate '01 a, p. 497, believed the 'hairs' and 'thorns' of the chiton 

 girdle to be sensory organs). 



' Heath ('99, p. 637; sep., p. 62) has described the use of the anterior flagella 

 of the transforming larva as tactile organs. 



