Phyhgeny of the Arachnida. 309 



appendages of the Crustacea and Arachnida, and in the other 

 Tracheata do not occur at all, is not without importance. In 

 cases in which structures of this kind are found in the latter 

 also it is not the modified terminal segment of the appendage 

 that plays the part of the movable joint, but a strongly deve- 

 loped thorn (a seta) ; in the contrary event, however, the 

 penultimate segment forms no projecting process. 



The Acarina are undoubtedly Arachnida, although greatly 

 modified and divergent from the primitive type. In this diver- 

 gence are expressed distinct signs of degeneration, and not a 

 cessation in development, as Bernard (No. 5) considers. 

 Bernard regards the Acarina as a fixed larval stage of the 

 Arachnids (probably of the Aranea?) ; he finds, namely from 

 instituting a comparison between the segmentation of the body 

 in Tetranyclnis tiliarwn, Herm., and in the Spiders (p. 281), 

 that in the abdomen of the Acarina certain (seven) segments 

 are wanting. These are situated between the segment 

 bearing the genital aperture and the anal segment (in Tetra- 

 vyclms only a single segment is found here instead of the 

 eight segments in the Arane^e). Owing to the absence of 

 these seven segments Bernard demonstrates certain peculia- 

 rities in the internal organization of the Acarina ; he compares 

 the heart of the Acarina with that of the Araneaj, and finds 

 that in the former precisely that portion of the heart (the 

 first two chambers) is absent which in the Aranea lies in the 

 middle abdominal segments; he finds, too, that the same is 

 the case with regard to the alimentary canal and the ventral 

 ganglion. The Acarina differ from the Araneas only in a 

 quantitative, not in a qualitative respect. In Bernard's 

 opinion, his view on the subject of the Acarina is confirmed 

 by Winkler^s statement as to a provisional fourth pair of legs 

 {Gamasus) and the exclusively tubular, consequently, as 

 Bernard believes, more primitive, tracheee, the stigmata of 

 which are situated in the cephalothorax far towards the front. 

 My own observations upon the development of Ixodes have 

 shown that Bernard's theory rests upon a false basis. The 

 abdomen of the Acarina divides in the embryonic period into 

 a larger number of segments ; subsequently this number is 

 reduced through fusion, but not through incomplete develop- 

 ment ; the segmentation of the body in the adult Acarina 

 possesses no significance at all : further, the primitive genital 

 aperture does not correspond to the definitive one. Bernard's 

 considerations as to the form of the heart, of the alimentary 

 canal, and of the nervous system consequently cannot be 

 accepted. The author's view on the subject of the fourth 

 pair of legs I do not clearly understand ; his view as to the 



