Dermal Sense- Organs of the Crustacea. 309 



connective-tissue sheaths of the tei-minal cords, and partly to 

 the intermediate hypodermis-cells, but in no case justify the 

 assumption of the existence of a second anterior gang-lion. I 

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

 nexion with the Myriapods 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 antennas of the Chilo- 

 gnatlia 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 the other (first antenna of the 

 Daphnids and Phyllopods according to Leydig, first antenna 

 of Leptodora according to VV'eismann), or that one ganglion 

 was divided into two parts connected by nervous matter (large 

 or second antenna 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 beliind 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 whole 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 hairf. 



* Leydig, " Ueber Ampliipoden und Isopoden," Zeitschr. f. wiss. Zool. 

 30 Bd. Suppl., 1878; "Arte?nia salina und Branchipus staynalis," ibid. 

 3 Bd., 1851; * Naturgescbichte der Daphuiden,' 1800; " Geruchs- und. 

 Gehororgane der Krebse und lusecten," Avcliiv f. Anat. u. Pliys. 1855 ; 

 " Die Hautsinnesorgane der Arthropoden," Zool. Auz. 9 Jhg., uos. 222 and 

 223, 1886. 



t Among Insects the instances in wbicli only a single sense-cell 

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

 cases described and figured by me, occur chiefly in tlie sense-organs of 

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

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

 51 Bd., i. Heft) Weinland, 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 further says :— " That several ganglion-cells send out 

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

 Rath to be the more usual occm'rence in Insects, is at any rate not the 

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



