I THE ARTHROPODS A NATURAL GROUP 17 
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 1uto 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, 264. 
VOL. IV C 
