170 Proceedings of the Royal Physical Society. 



fingers, the creature instantly flexes its whole body on the 

 abdomen, and the greatest quickness is necessary in removing 

 one's fingers out of the reach of its claws. The invariable 

 position of the tactile setse of the abdominal somites outside 

 the fringing hairs is worthy of remark. While the appendages 

 of the head and thorax form a sort of tactile zone round 

 those parts of the body, the abdomen, though less well pro- 

 vided with tactile limbs, has also a tactile zone of a more 

 simple kind. 



Swimmerets. — On the coxopodite, which is very small, 

 there are no tactile setse ; on the basipodite there are a few 

 small irregular groups ; but on the endopodite and exopodite, 

 on which the arrangement is practically identical, both the 

 tactile and fringing setae are very numerous, and it is here 

 that their relation to other parts may most readily be made 

 out. The fringing setse are arranged in a single row along 

 the margin and across the apex, standing on their character- 

 istic pedestals. They have their flat surface in the same 

 plane as the flat surface of the swimmeret, and increase its 

 surface. Here the tactile setse are most numerous and 

 longest at the apex, and are present in about equal numbers 

 on both the anterior and posterior surfaces of the appendages. 

 They stand singly, as the cuticle is thin, and pass back a 

 short distance up the joint, the proximal ones being much 

 shorter than those more distally placed. This is the arrange- 

 ment on all the normal swimmerets. 



Experimental. — The swimmerets are sensitive, for if they 

 be rubbed or twitched when the abdomen is extended, the 

 creature flaps its tail vigorously, and the posterior pairs of 

 ambulatory legs are pushed backwards to remove the object. 



From the preceding detailed examination of the surfaces 

 of the crayfish, it will be seen that, though tactile impressions 

 are only received by certain parts of the body, and by some 

 parts more than others, yet these sensitive points are much 

 more widely distributed than was previously imagined. It 

 is most interesting to notice how the distribution of the 

 tactile organs varies in accordance with the needs of the 

 animal, and how it has been altered from the primitive type 

 as the circumstances of the creature's surroundings changed. 



