Bell, Reactions of Crayfish. 309 



mouth parts to over a hundred on the antennules. There was 

 no spherical membrane separating the nerve from the lumen of 

 the hair, and the terminal strand entered the hair and ended 

 freely along the lov^er tw^o-thirds. Hensen's group of short 

 unplumed hairs in the otocyst is not mentioned by Prentiss in 

 his discussion of Palaemonetes, Crangon and Cambarus. In the 

 otocyst of Carcinus he speaks of "group hairs," 200 in number, 

 which are "short, thick, and blunt, without a trace of fringing 

 filaments." These, however, have the same sort of innervation 

 as all the other hairs observed in the otocyst, and Prentiss con- 

 siders them "degenerate tactile hairs" which have lost their plum- 



From this review it will be seen that our knowledge of the setae 

 of decapod Crustacea is anything but satisfactory. All of the 

 authors discuss sense hairs and speak of different kinds of sense 

 hairs, leaving it to be inferred that there are other kinds of hairs 

 not sensory in function. Gulland distinguishes sharply between 

 sensory setae and fringing setae, but his criterion is the presence 

 or absence of a nerve fibril in the lumen of the hair. Moreover, 

 his sensory setae are all smooth hairs, whereas Hensen, vom 

 Rath, and especially Prentiss have shown conclusively that 

 at least some plumed hairs have a sensory function. Vom Rath, 

 on the other hand, admits the possibility (though his preparations 

 did not seem to support it) that all setae are sensory, and Prentiss, 

 though he does not say so, seems to incline to that view. If, 

 then, we follow Prentiss' account and say that all hairs are 

 sensory, that plumed setae supplied by a single nerve which does 

 not enter the lumen are tactile, and that smooth setae which are 

 supplied with nerve bundles entering the hair and ramifying 

 alono- its lower two-thirds are chemical, we must assume that 



O ... 



Leydig failed to observe those nerve ramifications owing to faulty 

 impregnation, that all of the hairs studied by Hensen were tactile, 

 and that Claus and vom Rath selected only smooth, chemical 

 hairs for histological examination, neglecting the tactile altogether. 

 Moreover, Gulland's description of "tactile" hairs and their 

 innervation tallies very well indeed with Prentiss' account of 

 chemical setae, so that we may assume the latter were what Gul- 

 land was describing. In that case, however, we shall have to 

 assert that chemical hairs are found in considerable profusion all 



