rhylogeny of the Arachnida, 291 



primitive trachea) were likewise distributed over the entire 

 body, jis in tht' case of Peripatus Edwardsii, Bhmch. ; it was 

 not until further development was in prof^ress that, for 

 reasons which have been stated with accuracy by von Kennel, 

 out of the numerous tufts only those were left which were 

 situated in the immediate nei^t^hbourhood of the appendaiijes. 



In the Arachnids, therefore, we find two entirely different 

 structures: — (1) The lung-sacs of the Scorpion and the 

 Aranea*, and (2) the tracheae of the Acarina (partly also those 

 of the ISolifuga^). The resj)iratory organs of the remaining 

 groups of Arachnids must be homologous either with the first 

 or with the second division ; and in this sense the solution of 

 the question as to the homology of the abdominal tracheae 

 of the Solifugaj is of especial interest. For if it should be 

 found possible to compare them with the lungs of the 

 iScorj)ions, it would serve to confirm the view which has 

 already been expressed, that the common ancestor of the 

 Acarina, and probably of certain other orders of Arachnids 

 also, possessed at the same time lungs (**. e. branchiie) in the 

 abdomen as well as primitive trachese distributed over the 

 entire body. On the other hand, if the abdominal tracheal of 

 the Solifugffi, like the trachete of the Acarina, have arisen 

 from dermal glands, we may assume that there is still a 

 possibility that the ancestral forms possessed two kinds of 

 resjnratory organs ; but we might then suppose that also in 

 the case of certain other Arachnids {e. (j. in the Phalaugidai 

 and the Pseudoscorpions) the abdominal tracheaj are not 

 homologous with the lung-sacs of the Scorpions. The solu- 

 tion of the question will be rendered possible by a precise and 

 com])arative investigation of the development of the abdo- 

 minal and thoracic tracheaj of the Solifugai. 



Whatever this solution may be, it may be assumed, on the 

 basis of the considerations already stated, that the Arachnids 

 divided into two branches at an early period : in the one 

 group the trachece hecavie very strongly developed^ ichile the 

 tuny-sacs {the modijied brauchia') either in part persisted or 

 entirely disajipeartd ; in the other, the dermal ylands trans- 

 Jurmed into trachece altogether aborted, ivhile in compensation 

 for this the lung-sacs underwent very great development {cf. the 

 division founded upon the differentiation of the abdomen into 

 two sections, JSo. GG, p. 157). 



Since we are discussing the tracheai of the Acarina, the 



dermal glands in Atax wfxs described long ago by Claparede (loc. cit.) ; 

 each group of these unicellular glands has a common excretory duct, 

 which opens by a stigma-like oritice. 



