Bell, Reactions of Crayfish. 307 



seems to have escaped the notice of the German investigators, 

 divides the setae of the crayfish into two classes according to 

 wiiether the lumen receives a nerve fibril into it or not. The hairs 

 that have such an entering nerve fibril he calls "sensory setae," 

 those that have not are denominated "fringing setae," since they 

 usually form the fringes on the various appendages and segments 

 of the body. The sensory setae are tactile, auditory, or olfactory, 

 and he devotes himself at once to the consideration of the tactile. 

 These, he says, are rather long, smooth, hollov^^, and very elastic, 

 partly on account of their substance and partly due to the mode 

 of their attachment. This is as follow^s: "The lumen is widest 

 at the base, as a general rule, and the walls thickest; suddenly the 

 thick cylinder formed by the walls widens out, and the walls 

 become thinner and more membranous, and roof in a dilation of 

 the canal which pierces the integument; w^hile the membrane 

 itself passes up to become continuous with the cuticular thickening 

 or 'areola' which surrounds the base of the seta, and rises slightly 

 above the level oi the cuticle." A little less than half way up 

 the seta there is a joint-like appearance, and from here the hair 

 tapers to a point, or swells out again into a small knob which has 

 a slight depression at the tip like an elephant's proboscis. The 

 lumen extends almost to the point, but there is no opening. The 

 nerve fiber enters the lumen but does not pass all the way up the 

 seta; rather it is attached or opposed to an oval surface a very 

 short distance from the base of the seta, but distinctly within the 

 lumen. As to the nature of this attachment to the oval surface, 

 GuLLAND could give no further account. The fringing setae, 

 on the other hand, have the lumen closed near the proximal end 

 by a thick chitinous ingrowth, and there is no nerve supply. The 

 fringing setae are invariably plumed. Gulland gives a minute 

 and extended account of the distribution and arrangement of the 

 tactile setae, to the effect that they are found on almost all parts 

 of the body, except the eye-stalks and the carapace. He says that 

 only the setae on the middle segments of the antennules are olfac- 

 tory, those at the distal end being tactile. The arrangement of 

 tactile setae on the mouth parts agrees quite closely with that of 

 Bethe indicated above. 



The most recent as well as the most lucid account of sensory 

 setae in decapod Crustacea, so far as my know^ledge goes, is that 



