PHYLUM ARTHROPODA 627 



N 



RELATIONSHIPS OF THE AIR-BREATHING ARTHROPODA^ 



Notwithstanding the existence of some striking superficial 

 resemblances between the Arachnida and the Insecta, the evidence 

 afforded by anatomy and embryology points to the conclusion that 

 there is no direct genetic relationship between the two groups. 

 The occurrence in both of a peculiar form of respiratory organs, 

 the tracheae, seems at first sight to indicate such a relationship ; 

 but the evidence of an independent origin is so strong that it 

 must be supposed that the tracheae have been independently 

 developed in the two classes. The most important points of 

 difference are the separation of head and thorax in the Insecta, 

 the mode of development of the eyes, the presence in the Arachnida 

 of an extensive " liver " and (perhaps) the endodermal origin of 

 the Malpighian tubes in the latter class. 



Resemblances between Limulus and the Scorpions are readily 

 apparent. In both there is a cephalothorax bearing six pairs 

 of appendages, together with two median and several lateral 

 eyes. The appendages in both are all originally post-oral, the 

 first pair becoming pra3-oral in course of growth, and the ganglia 

 belonging to it coalescing with the brain. The upper lip be- 

 tween the bases of these appendages is similarly developed in 

 both. The pair of processes situated behind the sixth pair of 

 appendages, which in Limulus form the chilaria, are represented 

 in the Scorpions by a small pentagonal plate in front of the oper- 

 culum. The abdomen of Limulus corresponds to the prae- and 

 post-abdomen of the Scorpion ; it contains only eight segments : 

 but there is evidence, from a comparison with certain fossil forms, 

 that the last segment represents several united metameres. A 

 certain amount of correspondence is also traceable in the append- 

 ages of the abdomen. In both the first pair form the operculum : 

 in the Scorpion the second pair form the pectines, while the rest 

 disappear ; in Limulus all persist as the lamelliform appendages to 

 which the book-gills are attached. In structure there is consider- 

 able similarity between the book-gills of Limulus and the book- 

 lungs of the Scorpion, but how far they are equivalent to one 

 another remains doubtful in view of the difference in their 

 position, the book-gills being attached to the dorsal surface of the 

 abdominal appendages and the book-lungs sunk within the 

 segments. 



The presence in both of the large " liver," of a circum-oesophageal 

 artery, of a cartilaginous endosternite, and of a pair of coxal 



s s 2 



1 The Xiphosura and also the Pentastomida, though not air-breathing, are 

 I discussed here. 



