252 LESLIE B. AREY AND W. J. CROZIER 



elusion may be tested through the attempted physiological isola- 

 tion of chemical and tactile irritability. Such tests are, of 

 course, open to several sources of serious error; the most critical 

 results should be given by cases in which sensory exhaustion to 

 chemical stimulation did not interfere with tactile irritability 

 (Parker, '08, p. 440). A result of this kind is free from the objec- 

 tion that sensory fatigue may result in heightening the threshold 

 to the more delicately acting forms of activation. Such a result 

 is readily obtainable with Chiton: the ctenidia cease to respond 

 to chemical activation by 5/8 N NaCl after about ten trials at 

 brief intervals; they continue, however, to respond to touch. 



This finding strengthens the opinion already derived from the 

 distributional study of tactile and chemical activation in chiton. 

 The fact that the reactions induced by these modes of activation 

 are in some cases qualitatively identical, involving similar 

 muscular contractions, is no obstacle to this view. 



b. In the buccal cavity of chitons cup-shaped organs, with a 

 suggested 'gustatory function,' have been described, as well as 

 numerous nerve terminals in the subradular organ. We have 

 nothing on this subject to add to Heath's ('03) observations, 

 which we can confirm; these observations showed that positive 

 food-taking responses are initiated by the excitation of the ex- 

 ternal surface of the mouth region with the materials upon 

 which chitons feed. We have seen that mouth movements are 

 initiated by chemical activation. It will be of interest to dis- 

 cover to what class of substances chitons react by food-taking 

 responses. 



Some writers speak of an osphradium situated at the base of 

 each ctenidium in the chitons (Burne, '96). Pelseneer ('99, 

 p. 13) has described and figured the two erectile ridges on the 

 inner side of the posterior inner ventral border of the girdle (the 

 "lateral lappets of the mantle fold" of Plate, '97, pi. 2, fig. 

 15, Up.), immediately caudad of the gills (figs. 7 and 8); he con- 

 siders these structures to be homologous with the osphradia of 

 other mollusks. In some species these protuberances assume the 

 shape of well-defined papillae. They are situated on the distal 

 face of the pallial nerve cords, which supply them with a rich 



