Dermal Sense-Organs of tlie Crustacea. 307 



ir. IIlSTOLOOICAL StHUCTUKE OF TFIH NkUVE-EN'D 



ArPAHATUs OF THE Sensoky IIairs of Crustacea. 



'J'lic liistology of tlie nevve-eiid .'ippaiatus of tlic various 

 sciisory hairs, whether olfactory tubes or tactile liairs (smooth, 

 lialf-feathered, coni])letely feathered, or toothed), is es.^en- 

 tially the same, and corresponds most minutely with what I 

 liave previously described for Myriapods and Insects. J\Iy 

 interpretation of the finer structure of the nerve-end apparatus 

 of the sensory hairs of Arthropods differs somewhat from the 

 statements of other authors. 



In the Crustacea, beneath the l)aseof each capilliforra struc- 

 ture serving a sensory function, there lies a group of cells 

 which is connected with a nerve ; these cells are termed a 

 ganglion by authors ; but since they are manifestly the per- 

 cipient epithelial cells, I prefer to term them sense-cells, 

 without, however, intending thereby to insist on a strict 

 physiological distinction between ganglion- and sense-cells. 

 In very rare cases only, e. g. in the whole of the sensory 

 liairs of the cirriform feet of Lepas, I found beneath the hair 

 only a single bipolar sense-cell, of relatively large size and 

 elongate in form, with a roundish nucleus which considerably 

 exceeded the nuclei of the cells of the hypodermis in size. 

 According to the usually accepted view, tlie nerve which is 

 connected with the ganglion-cells is supposed to traverse the 

 entire length of the ganglion and then enter the sensory 

 hair. I have been able in a very large number of cases, e. g. 

 in the olfactory tubes of Astacus *, to convince myself with 

 absolute certainty of the fact that the nerve in no way passes 

 through the group of sense-cells, so that the sense-cells are 

 attached to the nerve-fibrils much as the grapes in a bunch ; 

 on the contrery, the nerve splits up beneath the group of 

 sense-cells and gives off a fibril to each cell. In the anterior 

 or distal region of the group of sense-cells I then distinctly 

 saw the way in which the protoplasmic prolongations of the 

 various cells unite into a finely streaked bundle, the terminal 

 cordj which actually enters the hair, while its tibrillate nature 

 can otten be distinctly recognized right to the tip of that 

 structure. Strictly speaking therefore the sensory hair does 

 not contain a true nerve, but rather the united prolongations 

 of sensitive epithelial cells ; it follows therefore that we can 

 scarcely speak of a true axis-cylinder or axis-fibre. The 



* An olfactory tube of Astacus, with the nerve-end apparatus belonging' 

 thereto, has aheady been described and figured by me in my previous 

 publication (Archiv f. mikr. Auat. 27 BJ., 1836). 



