Carpenter — Belafio)h'i/tip>y hetweeu Classes of Arthropoda. 339 



the hinder aspect of an abdominal appendage comparable to the gill of 

 Limulus. Purcell, in particular, has shown that the plates begin to be 

 developed while the appendage still stands out distinctly from the 

 ventral surface of the body ; as growth proceeds, the appendages 

 simply sink in, " without any inversion or other complications,'' as 

 Simmons remarks, and the lung-book is thus formed. And the 

 tracheal tubes in spiders arise as in-pushings behind the appendage of 

 the third abdominal segment, the appendage bearing evanescent fold- 

 ings resembling those that give rise to the lung-book on the appendage 

 of the second abdominal segment. Could stronger evidence be desired 

 that lung-books are more primitive among the Ai-achnids than tracheal 

 tubes, and that they were preceded by lamellate appendicular gills? 

 Moreover, Purcell shows that, in most Dipneumonous Spiders, part of 

 the tracheal system, and in the Attidee nearly the whole of it, arises 

 from the ectodermal in-pushings that form the entapophyses, so that 

 the tracheae have not, in all cases, a similar origin. Clearly it was 

 these entapophysial imaginations that gave rise to Jaworowski's 

 statement ('94) that the tracheal tubes precede the lung-books in the 

 development of Trochosa. It follows from these researches on the 

 development of Spiders' lungs and tracheae, that the latter are the 

 later development among the Arachnida, that they are not constant 

 in their mode of origin, and that they must have arisen altogether 

 independently of their origin among the Insects. 



These considerations show that the tracheal respiration of the 

 Solifugida cannot be invoked as an argument that the Arachnida as a 

 whole are " Tracheata," still less, as suggested by Thorell, that the 

 Solifugida are Insects ! For granting that, in their segmented fore- 

 body, the Solifugida have retained a primitive character lost by the 

 Scorpions, their abdominal segmentation is reduced and condensed as 

 compared with that of the latter, and their chelicerse are the most 

 powerful and specialized to be found in the whole class. The 

 existence of these very powerful limbs, and the extreme rapidity of 

 locomotion attained by these animals is sufficient to explain the 

 exceptional development of spiracles among them on the foni'th limb- 

 bearing segment of the cephalothorax. Even if, as Bernard ('96) 

 claims, these spiracles suggest the presence of breathing-organs on 

 nearly all the segments of the primitive Arachnid, there is no im- 

 possibility in such a conception. But the fact that cephalothoracic 

 breathing-organs are found only among the Mites and the Solifugida 

 — the former in all respects, and the latter in some respects, highly 

 specialized forms — suggests rather that breathing-organs among 



