306 Journal of Comparative Neurology and Psychology. 



sense hairs and ordinary ones by the criterion that the former 

 have sense cells at the root, the latter have not. Like Leydig, 

 he notes that the hair shaft consists of two parts, a proximal which 

 is thick and strongly chitinized, and a distal which is paler and 

 more delicate. He says that the only way to distinguish between 

 different kinds of sense hairs is by their position and their mode 

 of attachment. Besides the olfactory hairs on the antennules 

 there are many unplumed touch hairs, which are stiff and sharp 

 and situated on a strongly chitinized bulbous membrane. Touch 

 hairs may also be half plumed or plumed, but plumed hairs at- 

 tached to a delicate bulbous membrane are auditory hairs, whether 

 they occur in the otocyst or on the free surface of the body. There 

 is not one but several sense cells at the root of each hair, and the 

 nerve fibrils, after passing through these, unite again into a ter- 

 minal bundle which extends sometimes almost to the tip of the 

 hair. 



In Astacus Retzius ^ was unable to find any nerve entering the 

 hairs on the broad parts of the maxillae or of the abdominal 

 appendages. He is inclined, therefore, to disagree with Claus 

 and VOM Rath (in his earlier publications) regarding the inner- 

 vation of sense hairs in Astacus and Palaemon, but admits that 

 Cyclops shows the entrance of nerves into the antennular hairs. 



Bethe^ agrees that for most of the hairs on the mouth parts 

 and pleopods of Astacus it would be impossible for the nerve to 

 penetrate the hair, since there is no place for it to get through the 

 chitinous membrane on which the hair sits. But in certain places 

 on the mouth parts he finds hairs where the nerve does ramify on 

 the inside of the hair. The chief places are on the edge of the 

 endopodite of the second maxillipede, the edge of the larger palp 

 of the first maxillipede, and the edges of both palps of the first maxilla. 

 These, as well as the rest of the mouth parts, are also thickly cov- 

 ered with hairs which admit no nerve into their interior. These 

 are always plumed, while the innervated hairs are never plumed. 



GuLLAND,^ whose paper on "The Sense of Touch in Astacus" 



* Retzius, G. Das sensible Nervensystem der Crustaceen. Biologische Untersuchungen, Vol. 7, 

 pp. 12-18. 1895. 



^Bethe, A. Ein Beitrag zur Kenntniss des peripherischen Nervensystems von Astacus fluviatilis. 

 Anatomischer Anzeiger, Vol. 12, pp. 31-34. 1896. 



^ GuLLAND, G. L. The Sense of Touch in Astacus. Proceedings of ihe Royal Physiological Society, 

 Edinburgh, Vol. 9, pp. 1 51-179. 1886. 



