148 THE WAYS OF BREATHING 



the Molluscan phylum, that is to say, the cten- 

 idium is monophyletic. 



The spirally thickened air - tubes or tracheae 

 of Myriopods, Insects, and Spiders bear so close 

 a correspondence in structure and function, rami- 

 fying through the body like blood-vessels, but 

 effecting the circulation of air instead of blood, 

 that all the air - breathing Arthropods were 

 formerly classed together as Tracheata in con- 

 tradistinction to the aquatic Arthropods which 

 were called Branchiata. 



For more than a quarter of a century it has 

 been recognised that the Tracheate Arthropods 

 could not be reduced to a common standard, and 

 it has also come to be realised that the tracheae of 

 Insects and Arachnids have had separate origins, 

 and are therefore different morphologically though 

 similar histologically and physiologically. 



This fundamental example of tracheal con- 

 vergence is rendered more remarkable by the fact 

 that even within the limits of the Arachnoid sub- 

 phylum, the tracheae have had at least a twofold 

 origin, namely, from lung-books and from ecto- 

 dermal tendons; so that "similarity of structure 

 in the fully developed tracheae does not mean 

 similarity of origin " [Purcell]. Purcell has shown 

 that the tendinal or medial tracheal trunks in 

 Dipneumonous spiders are equivalent in their 

 entirety to metamorphosed entapophyses (ecto- 

 dermal tendons) ; the lateral tracheal trunks, on 



