Dennal Sense- Orcjans of the Crustacea. 309 



connective-tissue slieatlis of the terniiaul cords, and partly to 

 tlie intcruiediate hypodcrinis-cells, but in no case justify the 

 assuin[)tiou of the existence of a second anterior gani^lion. I 

 Mould remind the reader that I have already proved, in con- 

 nexion with the JMyriapods and Insects, that in all cases in 

 which authors, e. g. Sazepin, have described two ganglia 

 lying one behind the other, e. g. in the antennte of the Chilo- 

 gnatha and the Wasp, in reality only a single group of sense- 

 cells exists. In a similar way I convinced myself in the case 

 of the Crustacea that in those instances in which it was stated 

 by authors that the nerve-end apparatus consisted of two 

 ganglia lying one behind tlic other (first antenna of the 

 JJaplniids and Phyllopods according to Leydig, first antenna 

 of Leiitodora according to VVeismann), or that one ganglion 

 was divided into two parts connected by nervous matter (large 

 or second anteniui of the Woodlice according to Leydig *), in 

 reality only one ganglion, that is a single group of sense-cells, 

 is to be found ; and that hypodermis-cells have been mistaken 

 for a second distal ganglion. Moreover we may get the false 

 appearance of two groups of sense-cells lying one behind the 

 other, owing to the fact that tactile hairs also are usually 

 found in the immediate neighbourhood of the olfactory tubes, 

 and that, even in sections, the group of sense-cells belonging 

 to the former are always closer to the hypodermis than those 

 of the latter. We find the most interesting structural con- 

 ditions of the nerve-end apparatus among the Entomostraca. 

 I have already remarked that the wdiole of the sensory hairs 

 of the cirriform feet of Lepas show only a single large sense- 

 cell beneath their base, while hitherto in all other cases I have 

 always found a group of sense-cells beneath the sensory iiairf. 



* Leydig, *' TJeber Ainpliipoden iind Isopoden," Zeitschr. f. wiss. Zool. 

 30 Bd. Suppl., 1878 ; "Arte)nia saltna und Branchipus staffnalis," ibid. 

 3 Bd., 1851; ' Naturgeschichte der Daphuiden,' 1860; " Geruchs- und 

 Gehcirorgane der Krebse und lusecten," Archiv f. Anat. u. Phys. 185o ; 

 " Die Hautsinuesorgane der Arthropoden," Zool. iVuz. 9 Jhg., nos. 222 and 

 223, 1886. 



t Among Insects tlie instances in wliich only a single sense-cell 

 belongs to a hair are also by far the most unusual, and, in addition to tlie 

 cases described and figured by nie, occur chietiy in the sense-organs of 

 the halteres of Diptera, as has recently been shown by Weiuland. In 

 his paper on the balancers (halteres) of Diptera (Zeitschr. f. wiss. Zool. 

 51 Bd., i. Heft) "NVeinland, among other things, describes the histology of 

 the sense-organs belonging to the halteres, and states that, in connexion 

 with each of these different sense-organs, a bipolar ganglion-cell is always 

 found. Weinland fm'ther says : — " That several ganglion-cells send out 

 from among them only a single nerve-ending, as has been stated by vora 

 Rath to be the more usual occurrence in Insects, is at any rate not the 

 case in the nerve-end apparatus of the halteres ; Kiinckel's view is in this 



