Morphology of iJie Arachnida. 17 



be less likely to get blocked by sand than those of fciie 

 abdomen, owing to their being removed away from the sur- 

 face of the ground, and only very much exposed when the 

 coxae between which they lie are separated by the movement 

 in the vertical plane of the posterior half of the cephalo- 

 thorax. 



In some Pseudoscorpiones, too, the stigmata have also 

 changed their position, but instead of moving backwards as 

 in the Solifugai they have migrated externally from the 

 sternal plates to the lateral membrane of the somites. In this 

 situation they are much more freely exposed to the air and 

 much less likely to be blocked by sand or earth or other 

 foreign bodies. 



In the Opiliones the stigmata of the remaining pair of 

 breathing-organs retain their position in the middle of the 

 sterna of the somite to which they belong. 



From what has been already said concerning the affinities 

 of the orders of Arachnida, it will be seen that the replace- 

 ment of pulmonary sacs by trachcEe has taken place indepen- 

 dently at least twice — once in the Dipneumonous Spiders, 

 and once in e. g. the Pseudoscorpiones. This fact goes a long 

 way towards weakening the evidence of affinity between the 

 Opiliones and Pseudoscorpiones, not to mention the Solifugas, 

 on the grounds of similarity in their breathing-organs. For 

 the replacement may have been independently brought about 

 in the three cases. But however this may be with regard 

 to the three orders just mentioned, the fact that these tubes 

 have been developed twice in the same group bears very 

 strong evidence as to their efficacy as breathing-organs. 

 They must in fact be better adapted for their purpose than 

 the lung-book trachese. Perhaps the following considerations 

 may throw some light upon the matter. It seems that an 

 Arachnid furnished with tracheal tubes, such as Gahodes^ 

 must be considerably lighter and consequently more agile 

 than one, like a Scorpion, which possesses pulmonary sacs. 

 The loss of weight will be due to two causes : firstly, to the 

 fact that there will be a much greater supply of air inside the 

 body, owing to the ramifications of the air-tubes through the 

 tissues ; and, secondly, to the fact that the blood will be re- 

 duced in quantity, for there will no longer be need for a 

 rich supply of it, since the oxygen will be carried directly 

 to all parts of the body by the branches of the tubular 

 tracheae. 



Ann. & Mag. N. Hist. Ser. 6. Vol. xi. 2 



