Mr Gulla7id on the Sense of Touch in Astacus. 169 



posterior margins of the pleura there is a slight re-entering 

 of the angle, and from this depression — that is to say, on 

 the very edge of the free margin above and exterior to the 

 fringing setse — springs a single group of very stout tactile 

 setae, generally not exceeding six in number. 



The Caudal Fan. — By this name we may speak of the 

 swimmerets of the 6th pair and the telson, and here we find 

 a very great development of the tactile setse. Taking the 

 telson as an example, we find at the posterior margin an 

 irregular row (perhaps really two rows) of fringing setse, 

 which are very long and strong, and dorsal to them, and 

 therefore exterior when the abdomen is flexed or semiflexed, 

 two irregular rows of tactile setse, not arranged in groups, but 

 springing in an almost continuous series. The two rows 

 differ in size ; those of the proximal or dorsal row are short, 

 like those of the groups ; those of the distal or more ventral 

 row are long and stout. The same arrangement is repeated 

 on the endopodites and exopodites of the 6th pair of swim- 

 merets. The fringing setse pass up for some distance along 

 the margins of all the elements of the fan ; the tactile setee 

 are altogether confined to the free edge of the fan ; and on 

 the exopodite they are not found beyond the point where, 

 in the ordinary state of extension, it is overlapped by the 

 endopodite ; while on the telson these characteristic marginal 

 tactile setae cease from the point where it overlaps the endo- 

 podite. Thus there is a complete fringe of tactile setae along 

 the free margin of the caudal fan. On the under surface of 

 the caudal fan there are a few small groups. 



Experimental. — The pleura are sensitive at the extremities, 

 and the tufts there are brought close together when the 

 abdomen is flexed (its usual position) ; there is thus no 

 overlapping. Lemoine {loc. cit.) says that " les palettes de la 

 queue " are especially sensitive, and remarks that they have 

 a fringe of hairs which are very susceptible to tactile impres- 

 sions. This observation I have frequently confirmed, and 

 the presence of the numerous tactile setae fully explains it. 

 I may add, to show the extreme sensitiveness of this part, 

 that if, when the animal is lying quietly on its back, any 

 part of the edge of the fan be simply pressed with the 



