1883.] Is the Group Arthropoda a valid one? 1035 
those animals which in their adult state possessed certain features . 
in common were grouped together ; but with the study of embry- 
ology and the introduction of the principles of evolution, this is 
gradually being set aside and only those forms are associated 
which are shown to have had a common ancestry and a common 
descent. Thus classification will eventually be founded on gen- 
etic relationships, and not primarily on analogies of structure and 
accidents of form produced by similarity of environment and 
similar causes without a common ancestry. 
At first sight the homologies between the two groups of Ar- 
thropoda, Crustacea and Tracheata, seem very evident and easy 
to trace, the two groups appearing to be closely related. We 
find in each the same general features, a jointed body, each seg- 
ment of which is, to a certain extent, a repetition of its imme- 
diate neighbors. To this jointed body are attached a varying 
number of jointed appendages each modified for the purpose 
of feeding, locomotion or reproduction. A straight alimentary 
canal traverses the body as an axis, and above it is found the 
dorsal vessel or functional heart, while on the floor of the body 
Cavity is found the nervous cord, consisting of a series of ganglia 
connected by commissures, and in each group, when the cesopha- 
gus is reached, a commissure passes on either side connecting 
the ventral chain with the brain or supra-cesophageal ganglion. 
Thus far our knowledge, derived from comparative anatomy, 
Seems conclusive, but when we attempt to trace homologies far- 
ther, we"become entangled in a snarl which we think cannot be 
untangled except by heroic treatment. 
In the Crustacea the eyes, two pairs of antennz and the simple 
Median eyes of the young of many forms are innervated from the 
Supra-cesophageal ganglion; in insects one pair of antennz are 
entirely absent, and we have nothing to indicate whether the pair 
which exists corresponds to the antennz or to the antennulz of 
the Crustacean. In the insect the post-oral appendages of the head 
are three, mandibles, maxilla and labium; in the Crustacean we 
find the same number of cephalic appendages, mandibles, first and 
Second maxillze, but beyond this we cannot carry our homology 
in a serial order. And further, the appendages themselves in the 
two groups show very marked and important differences. In the 
Crustacea the typical structure is biramous; we have a basal 
Joint bearing two jointed branches. These parts to be sure are not 
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VOL. XVII,—No, x, 
