PhylogeMy of the Arachnida, 287 



ancestor of the Aracliuids possessed the cephalothoracic 

 tracheje, or whether they have been developed independently 

 within the class. The solution of this question is of the 

 highest interest from the theoretical point of view, since the 

 circumstance that the tracheaj, although only in the case of a 

 portion of the Arachnids, arise independently in certain seg- 

 ments, and have not been inherited from the common ancestor 

 of the entire class, constitutes an important, albeit a negative, 

 argument against grouping the Arachnids with the rest of the 

 Tracheata ; for it is tantamount to admitting that a charac- 

 teristic feature of the whole of the Tracheata, which, according 

 to von Kennel (No. 24), is sufficient to prove the common 

 origin of any two groups of Arthropods, can arise indepen- 

 dently. It seems to me that, in the present state of our 

 knowledge as to the development of the Arachnids and the 

 structure of the simplest members of the class, we can return 

 a fairly definite answer to this question, to the effect that 

 the ancestor of the Arachnids had no trachem in the cephalo- 

 thorax. The entire facts point to the conclusion that the 

 Scorpion is the most ancient form, standing nearest to the 

 ancestors of the Arachnids ; and yet neither in its structure 

 nor in its ontogeny do we tind the slightest indication that 

 it ever possessed tracheee which opened upon the cephalo- 

 thorax. Secondly, we find cephalothoracic stigmata in the 

 case of that somewhat divergent order the Solifugaj, although 

 in this instance abdominal stigmata likewise occur. Thirdly, 

 in the Acarina, which in organization have diverged furthest 

 from the primitive type, the abdominal stigmata have already 

 been entirely lost. From these considerations we can deduce 

 two conclusions : — (1) The tubular tracheaj of the Arachnids, 

 the stigmata of which are situated upon the cephalothorax, 

 are not directly homologous witli the tracliege of the rest of 

 the Tracheata, since they have been developed after the 

 Arachnids branched off; (2j the appearance of the stigmata 

 upon the cephalothorax is connected with the disappearance 

 of those upon the abdomen. This latter fact is perhaps to 

 be explained as being due to the considerable development 

 of the muscles in the cephalothorax of these forms : on the 

 one hand, with increased muscular activity a considerable 

 destruction of organic matter takes place, while on the other 

 the contraction of the muscles promotes aerial ventilation in 

 the tracheal {cf von Kennel, No. 24, p. 30, on the signifi- 

 cance of the position of the stigmata close to the appendages) ; 

 besides this, the relatively feeble development of the abdomen, 

 which is greatly reduced in the case of the Acarina, has had 

 as a consequence the disappearance of the stigmata thereon ; 



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