334 SCIENCE PROGRESS. 



appendages, for there is every reason to believe that the 

 long genital process of the former is really derived from a 

 pair of limbs. The limbs of the second segment persist 

 only as scales covering the apertures of the tracheae in 

 Galeodes, but in Scorpio they are still more or less leg-like 

 and are specialised as sensory appendages — the "combs". 

 Further, we have spinnerets, two pairs in the spiders. 

 These are very large and jointed like legs in some forms. 

 And lastly, we have the sting of Scorpio which is perhaps 

 best accounted for as having been formed out of the limbs 

 of the last segment, fused posteriorly over the anus. This 

 would at least account for the pair of poison glands with 

 two distinct ducts and apertures. 



Specialisation of the Respiratory Invaginations. — An in- 

 teresting problem is presented by the respiratory system of 

 the ancestral Arachnid. What was it? We have at least three 

 distinct types of respiratory cuticular invaginations in exist- 

 ing Arachnids. As the most highly specialised we have what 

 are known as book-lungs, because the air runs in flat spaces 

 separated by chitinous laminae within which the blood 

 circulates ; these are arranged alternately like the leaves of 

 a book. Such localised lungs necessitate a highly organised 

 circulatory system. They are found in Scorpions, Pedipalps 

 and Spiders. A second well-marked kind of respiratory 

 invagination occurs in the Book-scorpions, Harvestmen 

 and some Mites. It is a simple tubular invagination, the 

 inner end of which widens and then breaks up into an 

 enormous number of long fine tubules carrying air into 

 the remotest parts of the body. These are called tuft- 

 tracheae. Lastly we have the ordinarily branching tracheal 

 tubes which are best developed in the Galeodidae. 



These three forms of respiratory invaginations must 

 certainly be regarded as modifications of some simpler form, 

 from which they could all be derived. I say "must" be- 

 cause they all open in exactly the same association with 

 limbs or limb-vestiges and are thus certainly homologous 

 structures. There can, I think, be little doubt that these 

 different forms of tracheae are due to the different ways in 

 which the alimentary canal has developed under the influence 



