Bell, Reactions of Crayfish. 305 



many as 35 segments, 15 or 16 of which may bear olfactory setae. 

 C. affinis may have 33 segments with 19 bearing olfactory setae, 

 and C. blandingii has as many as 50 segments, 29 with olfactory 

 setae. Thus Wright's contention is not supported by the facts. 

 Faxon states, however, that the olfactory setae of C. pellucidus 

 are longer than those of most species of Cambarus. 



In his study of the otocyst of decapod crustacea Hensen^ states 

 that the auditory hairs differ from the touch hairs found in the otocyst 

 in that the former do not spring directly from the walls but are raised 

 on a small thin spherical membrane which permits great delicacy 

 of movement. Three classes of auditory hairs are distinguished: 

 (i) Otolith hairs, which surround the otoliths, and are more or less 

 completely plumed; (2) free hairs in the otocyst, also called hooked 

 hairs on account of being bent at a right angle near their middle, 

 heavily plumed; (3) free auditory hairs on the surface of the body. 

 These are also plumed. Besides these there is a little clump of 

 hairs in the otocyst which are short and unplumed. Hensen 

 thinks the auditory hairs are rather sharply differentiated from 

 the tactile hairs, chiefly by the spherical membrane at the base. 

 In all sense hairs he finds a peculiar process, the lingula, extending 

 into the central portion of the base, and to this the nerve is 

 attached. 



Claus,- discussing sense hairs in general but referring to olfac- 

 tory setae in particular, disagrees with Leydig in two points: (i) 

 he affirms that the sense hairs are never open at the tip, and that 

 any such appearance is due to their being broken off; (2) he asserts 

 that the sense hairs are supplied with a single nerve each and 

 that this nerve can be shown, in favorable specimens, to enter the 

 hair — a thing which Leydig had denied. Vom Rath^ confirms 

 Claus' view that sense hairs are penetrated by the nerve, but is 

 unable to say whether they are always closed by a membrane or 

 not. In any case he showed by immersing the animals in methylene 

 blue that the liquid gradually penetrated the interior of the olfac- 

 tory setae even when closed. Vom Rath distinguishes between 



Hensen, V. Studien iiber das Gehororgan der Dekapoden. Zeitschrift fiir wissenschajlliche 

 Zoologie, Vol. 13, pp. 319-412. 1863. 



-Claus, C. Ueber das Verhalten des nervosen Endapparates an den Sinneshaaren der Crustaceen. 

 Zoologischer Anzetger, Vol. 14, pp. 363-368. 1891. 



^ Vom Rath, O. Zur Kenntniss der Hautsinnesorgane und des sensibeln Nervensystems der Arthro- 

 poden. Zeitschrift fiir wissenschaftliche Zoologie, Vol. 61, pp. 499-539. 1896. 



