Mr Gulland on the Sense of Touch in Astacus. 155 



Of these older writers, tlie papers of Leydig and Weismaiiii 

 on the larva of Corethra already cited, and Leydig's papers,^ 

 are especially worthy of notice, but the want of physiological 

 confirmation detracts very much from the value of all these 

 contributions. I shall again have to refer to Grobben's 

 paper ^ and to Studer,^ and so content myself at present with 

 mentioning them. 



Tlie tactile nerve end-organs which have so far been studied 

 in the Crustacea have been the simple forms, which are found 

 in the lower members of the group ; the mode in which the 

 nerve-fibre is connected with the seta had not been satisfac- 

 torily made out, and there was a tendency to regard the nerve 

 as passing to the end of the seta ; the distinction between the 

 various forms of setae was imperfectly known, as was also the 

 relations between those of higher and lower forms; and the dis- 

 tribution of tactile set^ had nowhere been ascertained clearly. 



[I can only mention Professor Leydig's interesting paper 

 (Die Hantsinnesorgane der Arthropoden Zool. Anzeig. 222, 

 223, May 1886), and regret that I cannot agree with his view 

 of the contents of the tactile setse. It seems to me that the 

 mode of formation of these setse, as well as direct observation 

 of the facts, quite excludes the possibility of the contents 

 being nervous in character.] 



Forms of the Set^ on the Exoskeleton of Astacus. 



As Braun {loc. cit., p. 131) has already pointed out, these 

 may be divided into two main groups, according to whether 

 the lumen of the seta communicates with the canal through 

 the integument over which it stands or not. Those in which 

 there is this communication seem in every case which has 

 been examined to be in connection with a nerve-fibril, and 

 may be called " sensory setce" and, according to the peculiar 

 form of sensory impression which they are modified to re- 



1 Zur Anatomie der Insecten, Eeicliert und Du Bois Reymond's Archiv fur 

 Anatomie, etc., 1859. 



2 Ueber blaschenforuuge Sinnesorgane und eine eigenthiimliche Herzbildung 

 der Larve von Ptyclioptera contaminata, Sitzb. d. Akad. d. Wiss. zu Wien, 

 Bd. Ixxii., 1875. 



^Nervenendigmigen bei Insecten, Mittli. der Berner Naturforscli. Gesellsch., 

 1873. 



