Mr Giilland on the Sense of Touch in Astacus. 165 



generally rather straighter than those, for instance, on the 

 abdominal appendages. 



Outside most of the groups of setse, and standing slightly 

 apart, are to be seen one or two fringing setoe., whose loss of 

 function has caused a peculiar modification. They are shorter 

 than the tactile setai (PI. VIII., Fig. 2), cylindrical, with 

 walls much thicker than in the normal fringing seta ; have 

 their lumen closed, are rounded off at the tip, and have their 

 secondary bristles twisted and twined round the main stem 

 of the seta, so that they look like fringing setse which have 

 endeavoured to become tactile, but by reason of the closure 

 of their lumen, and their having been already too much 

 specialised in another direction, have failed to do so. Their 

 presence is interesting, inasmuch as it illustrates the evolu- 

 tion of what was originally a swimming appendage into a 

 tactile limb. 



These groups of tactile setae are especially large and 

 numerous towards the tip of the " pincers ; " there are a 

 considerable number along the outer sides of both joints of 

 the " hand," and a few small groups are scattered over the 

 surfaces, being more frequent on the rough surface, which is 

 usually turned upw^ards and outwards, and less frequent on 

 the smoother inner surface. On the carpopodite the arrange- 

 ment is similar; on the meropodite the setse are almost 

 confined to the two serrated edges ; on the ischiopodite and 

 basipodite there are a few very fine setae, and very few also 

 on the dorsal surface of the coxopodite. On the ventral 

 surface of the coxopodite, however, the tactile groups are 

 numerous, especially on the proximal side of the articulation 

 with the basipodite. The setae which Huxley {loc. cit., p. 78) 

 calls " coxopoditic " seta, and which project into the branchial 

 chamber, as well as those on what he- calls the "base of the 

 podobranchia," are merely fringing setae peculiarly modified ; 

 they are longer and more delicate, and have their secondary 

 bristles thicker and at greater intervals than the normal 

 fringing setae. 



The description of the arrangement of the setae given 

 above applies perfectly to the 5th, 6th, 7th, and 8th thoracic 

 appendages (two lesser claws, two posterior ambulatory legs) ; 



