304 Dr. 0. voin Eatli on (lie 



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 the 

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

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

 author distinguishes otolith-hairs, free hairs in the auditory 

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

 Characteristic for all auditory hairs is their mode of attach- 

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

 upon an extremely delicate cupola- or dome-shaped membrane, 

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

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

 sound. According to Hensen, " the auditory liairs stand 

 npon 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, occu])ying an intermediate position between 

 typical, freely mobile, auditory hairs, and feathered, stitf, 

 unmistakably tactile hairs, resting upon a strongly chitinized 

 cu])ola-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 antenna; unfeathered, half-feathered, 

 com])letcly 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 fiagelliform 

 and bears the typical olfactory tubes, while the other is 

 expanded into a squamiform plate, the margin of which is 

 beset with 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 

 antennae of certain Amphipods peculiar hairs have been found, 

 the so-called ^^ caJceoU.''^ These shoe-like appendages, the 

 physiological importance of which is still obscure, are by no 

 means confined, as was formerly supposed, to the fiagellum 

 of the lower 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 antennje. 



The sensory liairs of the second antenna are of far less 



* Ilonsen, *' Studien iibor das Geliuroi-jrau dcr Pocnpodiii," Zeit.<cbr. 

 I'lir wiss. Zuol. lo l!d., l60o. 



