1894.] The Classification of the Arthropoda. 123 
they have no common ancestor nearer than the Annelids. 
The jointed nature of the appendages offers no insuperable 
objection to this view, while the early phases of the egg, the 
formation of the germ layers, the structure of the alimentary 
canal, the morphology of the reproductive and excretory or- 
gans, as well as certain facts concerning the circulatory, respir- 
atory and nervous systems are easiest explained upon such an 
hypothesis. The presence of compound eyes in branchiate 
and tracheate forms would, at first thought, be a strong argu- 
ment for the older views, but these organs differ so greatly in 
their structure that it is easier to regard them as homoplastic 
organs (comparable in a way to the eyes of Cephalopods and 
Vertebrates) rather than as derivatives from a common com- 
pound ancestral visual organ. For our present purposes, the 
group of Arthropoda may be retained as a convenient assem- 
blage, characterized in the following manner: Heteronomously 
segmented animals, with, typically, a pair of appendages to 
each somite; the whole enclosed in a chitinous segmented exo- 
skeleton, the jointing of which extends to the appendages, 
thus justifying the term Arthropoda. The appendages, prim- 
itively locomotor in function, may be modified, on one or more 
somites, for the taking or commuting of food, for respiration, 
copulation, oviposition, sensation, fixation, etc. No circular 
layer of muscles in body wall; nervous system consisting of a 
pair of primitively supracesophageal ganglia and a ventral 
chain of paired ganglia, of which one or more pairs may, in 
the course of development, be transferred to the prestomial re- 
gion. Eyes, simple, aggregate, or compound, with, in some 
cases, an inversion of the retinal layer. Colom small, incon- 
Spicuous; circulatory organs consisting of a dorsal heart en- 
closed in a vascular pericardial sac ; blood-vessels more or less 
evidently metameric, terminating in “lacunar” spaces. Respir- 
ation, either by the entire surface of the body or by specialized 
outgrowths or involutions of the same. Excretion, either by 
true nephridia or by Malpighian tubules, developed from 
either the mid- or the hind-gut. Reproductive organs consist- 
ing of gonads developed from the celomic walls and with 
modified nephridia serving as efferent ducts. 
