160 Proceedings of the Royal Pliysical Society. 



These have already been recognised by Lemoine and Huxley 

 in the crayfish, and by M'Intosh in Carcinus mcenas, and are 

 usually recognised in systematic descriptions of Crustacea. 

 It is quite possible to introduce a needle between the pincers 

 without the creature taking any notice of it, but as soon as 

 one of the tufts is touched, no matter how gently, the pincers 

 close at once with a snap more or less decided according to 

 the vigour of the animal, the strength of the impression, and 

 the number of tufts touched. If the tubercles on the pincers 

 be touched, there is no result so long as the tufts of setae are 

 not pressed upon. On examining the tufts with higher 

 powers of the microscope, I found that they were composed 

 of setae of the type which I have already described as tactile, 

 and the " index " of the propodite was also specially suited 

 for demonstrating the connection of nerves with the setae. 



From these data I considered myself justified in assuming 

 that these setae were specially adapted for touch, and pro- 

 ceeded in a similar way to make a detailed physiological and 

 microscopical examination of the seta3 of the surface of the 

 body. 



The Eyes. — On no part of either the peduncle, the surface 

 of the eye, or the membrane between the eyes, are there setae 

 of any kind. It will then, at first sight, seem strange that, 

 if the peduncle be touched, the eye is drawn back under 

 shelter of the rostrum, and the first pair of small thoracic 

 chelae brought forward to endeavour to remove the offending 

 object. This, however, is only done if the animal sees the 

 needle, and by using a curved needle from behind, and 

 touching the peduncle very lightly so as to prevent all chance 

 of pressing on any sensitive setae of the antennules, it will 

 be seen that there is no inherent sensitiveness in the peduncle. 

 It is of course impossible to say whether the surface of the 

 eye is sensitive to touch or no, as the eye is at once drawn 

 out of the way of any object which is seen threatening it. 



The Antennules. — As is well known, the exopodite bears 

 those setae to which Ley dig ascribes the sense of smell, and 

 the arrangement of which has been repeatedly described. In 

 addition to these, there is a ring of tactile setae set rather 

 far apart round the distal margin of each segment, the points 



