Mr Gulland on the Sense of Touch in Astacus. 159 



The closure of the lumen takes place at a j)oint about 0"03 

 mm. above the articulation with the integument, by means 

 of a chitinous plate about 0'006 mm. thick, which passes 

 right across the lumen. As the canal on which they stand 

 is relatively much wider than that of the tactile setse, the 

 hypodermis passes up nearly as far as the closing plate; and 

 this seems to confirm Braun's suggestion as to its formation. 

 The articulation differs slightly from that of the tactile setae, 

 inasmuch as the membranous part is not so long; the 

 " areola " is more delicate and nearer the base of the seta, 

 and the whole is raised on a sort of pedestal formed by a 

 projection of the integument. 



These setse vary considerably in details, but the type is 

 always recognisable. Sometimes the bristles are only found 

 on the distal part, or the seta is not quite so flat as I have 

 described it, and the setae are arranged more irregularly, and 

 so on. 



Distribution and Physiology of the Set^. 



The only writer who has attempted to examine the function 

 of the setae experimentally is Lemoine, and he has done no 

 more than commence the study, and has in addition made 

 but few and imperfect microscopical observations. I shall 

 now explain the grounds which led me to regard as tactile 

 those setae to which 1 have already ascribed that function. 



I examined first the antennae, since they have always 

 been regarded as peculiarly sensitive to tactile impressions, 

 but found that though the animal evidently assumed an 

 attitude of watchfulness, and showed some disquietude when 

 I rubbed the surface of the antenna or twitched it, yet that 

 there was no immediate response, and that, moreover, the 

 setae were so small and so thinly set, that it was impossible 

 to say whether the impression was received by them or by 

 the general surface. I then turned to the great claws, where 

 in the " hand," with the naked eye, it is perfectly easy to 

 distinguish delicate transparent projections, especially nume- 

 rous along the inner edges of both the dactylopodite and the 

 " index " or " anvil " of the propodite, which, with a low 

 magnifying power, resolve themselves into tufts of setae. 



