Mr Gulland on the Sense of Touch in Astaciis. 157 



the formation of the new seta, and, as Braun {loc. cit., p. 151) 

 points out, is drawn out with the seta in the process. Under 

 no circumstances is tliere any " nervous matter " in the 

 lumen. 



The setae are uncalcified, chitinous, and very elastic, which 

 is due partly to their own substance, partly to the joint 

 which connects them with the integument. They are bright 

 yellow in colour, and the stouter and thicker their walls, the 

 more this is marked, so that it is probably due to the actual 

 colour of the chitin. Sometimes, especially when the setae 

 are thick and strong, there is an appearance as if the wall 

 were double, which is indicated in PL VIIL, Fig. 1. The 

 manner of their articulation to the integument is peculiar, 

 and differs slightly from that of the fringing setse. The 

 lumen is widest at the base, as a general rule, and the walls 

 thickest; suddenly the thick cylinder formed by the walls 

 widens out, and the walls become thinner and more mem- 

 branous, and roof in a dilatation of the canal which pierces 

 the integument ; while the membrane itself passes up to be- 

 come continuous with the cuticular thickening or " areola " 

 which surrounds the base of the seta, and rises slightly above 

 the level of the cuticle. 



This will be better understood by reference to PI. IX., Fig. 

 12. The main use of the dilatation of the canal under the 

 setse is evidently to allow the base of the seta, which is 

 slightly sunk in the dilatation, to move freely. There are 

 slight variations in the forms of the setae, the most noticeable 

 being an occasional very minute serration of the distal por- 

 tion, whose morphological significance will be noticed else- 

 where. 



Fringing Setce. — These likewise vary slightly in their de- 

 gree of development in different parts, but are also best seen 

 on the swimmerets. Their most essential characteristics are 

 the absence of any connection with nerves, and the closure 

 of the lumen near the proximal end of the seta by a thick 

 chitinous ingrowth; this, according to Braun {loc. cit., p. 151), 

 is not present immediately after the moult, but is formed by 

 the new " papilla." In connection with this, it is interesting 

 to notice that in Thysanopoda the fringing setse are not 



