I THE ARTHROPODS A NATURAL GROUP 1 7 



may be able to form an opinion upon the position of the 

 Crustacea relative to other Arthropoda, and upon the question 

 debated some time ago in the pages of Natural Science ^ whether 

 the Arthropoda constitute a natural group. The Crustacea 

 plainly agree with all the other Arthropoda in the possession of 

 a rigid exoskeleton segmented into a number of somites, in the 

 possession of jointed appendages metamerically repeated, some 

 of which are modified to act as jaws ; they further agree in 

 the general correspondence of the number of segments of which 

 the body is primitively composed ; the condition of the body- 

 cavity or haemocoel is also similar in the adult state. An 

 apparently fundamental difference is found in the entire absence 

 during development of a segmented coelom, but since this 

 organ breaks down and is much reduced in all adult Arthropods, 

 it is not difficult to believe that its actual formation in the 

 embryo as a distinct structure might have been secondarily 

 suppressed in Crustacea. 



The method of breathing by gills is paralleled by the 

 respiratory structures found in Limulus and Scorpions ; the 

 transition, if it occurred, from branchiae to tracheae cannot, it 

 is true, be traced, but the separation of Arthropods into 

 phyletically distinct groups of Tracheata and Branchiata on this 

 single characteristic is inadmissible. On the whole the Crustacea 

 may be considered as Arthropods whose progenitors are to be 

 sought for among the Trilobita, from whose near relations also 

 probably sprang Limulus and the Arachnids. 



1 Vol. X., 1897, pp. 97, 234. 



VOL. IV 



