" WIND-SCORPIONSr 335 



of the force-pump action of the oesophagus, for the respira- 

 tion is always intimately associated with the circulation, and 

 it is this latter that would be affected first of all by any 

 great extension of the alimentary system. This subject 

 would, however, require more details for its elucidation than 

 the limits of this article would permit. I have dealt with it 

 in detail elsewhere. 



T/ie Nuuibc7' of Pairs of Stigniata. — Although, in 

 existing Arachnida, the respiratory invaginations are now 

 confined in each form to a very few segments (ac the most 

 four, Galeodes, Scorpio) we are fully justified in asserting 

 that, in their simple condition in the ancestral form, they 

 were present, one on each limb, throughout the whole 

 extent of the body. Specialisation of this diffuse condition 

 must very early have set in, indeed such limitation must 

 have proceeded concurrently with the differentiation of the 

 body into two regions. The restriction of the digestive 

 and generative processes to the abdomen would tend to 

 confine the circulatory and respiratory mechanisms also to 

 this same region. Hence, we find the respiratory invagin- 

 ations early disappearing from the anterior region. This 

 disappearance would be still further accelerated by the 

 tendency to knit the whole of the anterior region into a 

 rigid skeletal box specialised purely for the mechanism of 

 locomotion. It is strongly confirmatory of the whole line 

 of argument here used that the only Arachnid (omitting the 

 Mites) which has retained a pair of respiratory invaginations 

 in the anterior region is also the only Arachnid which has 

 never added any segments to the first three, the fusion of 

 which, as we have above endeavoured to show was the 

 initial specialisation which differentiated the class x'\rachnida 

 from their Annelidan ancestors. In all other existino- 

 Arachnids, except in certain Mites, the respiratory invagina- 

 tions have disappeared entirely from the anterior region. 



Those on the abdomen on the other hand seem to have 

 persisted for a long time as a complete series, although 

 here also they early began to degenerate progressively 

 from behind forward, so that they are now only found in 

 a few pairs on the anterior abdominal segments close 



