304 Dr. O. vora Rfvtli on the 



before commencing my experiments I was careful to ascertain 

 tliat all tlie olfactory tubes were intact. 



With the auditory organ situated in the basal joint of tlie 

 first antennae of the Decapods I shall deal very shortly, and 

 refer the reader to Hensen's * detailed description. This 

 autlior distinguishes otolith-hairs, free hairs in the auditory 

 sac, and free hairs situated upon the surface of the antennae. 

 Characteristic for all auditory hairs is their mode of attach- 

 ment, in that the shaft, wliich is always feathered, stands 

 upon an extremely delicate cu])oIa- or dome-shaped membrane, 

 in consequence of which the hair is able to swing to and fro 

 with the greatest ease, and can be set in motion by waves of 

 sound. According to Hensen, " the auditory hairs stand 

 upon a pore-canal, the walls of which develop on one side a 

 larger or smaller thickening, the tooth. All hairs exhibit at 

 one portion of their proximal end a peculiar process, the ligula, 

 to which the nerve is attached." Contrary to Hensen, in 

 examining my extensive material I not unfrequently met with 

 feathered hairs, occupying an intermediate position between 

 tj]ncal, freely mobile, auditory hairs, and feathered, stiff, 

 unmistakably tactile hairs, resting upon a strongly chitinized 

 cujjola-shaped membrane, so that it was a moot point whether 

 such transitional forms were to be regarded as auditory or 

 tactile hairs. 



Among tactile hairs, always ending in a sharp point, there 

 are found upon the first antennae unfeathered, half-feathered, 

 completely feathered, and toothed sensory hairs. 



In the first antenna of Nehalia there spring from a four- 

 jointed shaft two branches, of which the one is flagelliform 

 and bears the typical olfactory tubes, while the other is 

 expanded into a squamiform plate, the margin of which is 

 beset Avith a large number of long, fine, sharply pointed 

 sensory hairs, which are not plumose, but rather finely denti- 

 culate. 



Incidentally I would just allude to the fact that upon the 

 antennas of certain Amphipods peculiar hairs have been found, 

 ihe so-called " caZceo/^'." These shoe-like appendages, the 

 physiological importance of which is still obscure, are by no 

 means confined, as was formerly supposed, to the flagellum 

 of llie loAver antenna? of the male, but occur, as has been 

 shown by later investigations, in some forms in the female 

 sex also, and, moreover, on both pairs of antennas. 



The sensory hairs of the second antenna are of far less 



* Hensen, " Studien liber das Geliuiurgau der l>ecapudcn," Zeit.-clir. 

 fur Aviss. Zool. lo Jjd., IbUo. 



