274 M.T. Thorell on the Systematic Position of the Areulide. 
poda, if indeed they morphologically correspond with these. 
The eyes in Argulus are moveable, and agree in their structure, 
as is well known, with those of the Phyllopods in their most 
sharply defined form. 
In the Copepoda we never meet with the shell armature which, 
as a kind of supplementary integument on the anterior divisions 
of the body, in the Branchiopoda so generally encloses or covers 
the body. The large shie/d in the Argulidz shows the greatest 
resemblance to the carapace in Apus, and removes them far 
from the Copepoda. 
With regard to the internal structure, this in many instances 
affords a more uncertain criterion for judging of the affinity of | 
the lower Crustaceans than do the characters which are drawn 
from their external form. Thus the nervous system, the circu- 
latory apparatus, &c., in nearly related forms, may show very 
striking discrepancies, to adduce an instance of which we will 
only mention the known fact that, even in the strictly natural 
group formed by the gnathostomous Copepods, the Calanidze 
and Pontellidze possess a heart, while the nearly allied Cyclo- 
pide, with others, want a definite central organ of circulation. 
At the same time, however, that the internal structure presents 
such extraordinary variation and inconstancy even in the most 
nearly related groups, it shows, on the other hand, in its general 
features such striking correspondences in the Branchiopods and 
Copepods, that it is difficult to fix any anatomical character as 
belonging to one only of these two orders. The generative ap- 
paratus in the Copepoda only furnishes two peculiarities which 
are worth noticing here. The eggs are united in the Copepoda, 
when they leave the ovaries, to form one or two external egg-sacs, 
excepting in the spurious parasitic Notodelphyide and Bupro- 
ridz, in which they are received into a matria, almost as in the 
Cladocera. In the Branchiopoda such egg-sacs are never pre- 
sent. In the Copepoda, impregnation takes place, as far as we 
know, always by means of spermatophores, which are attached 
externally to the body of the female close to the sexual orifice 
or the mouth of a separate canal leading to the receptacle. The 
Branchiopoda never show such spermatophores. In both these 
points the Argulide depart from the Copepoda in general, and 
from the analogous forms in that order, Caligidze and Ascomy- 
zontide in particular, in that they lack both egg-sacs and ex- 
ternal spermatophores. 
Notwithstanding the agreement which the Argulide present 
with the Branchiopoda in respect to conditions of propagation, 
they yet exhibit in this very direction both peculiarities and 
analogies with other orders. Zenker has already pointed out 
this: in respect to the form and products of its generative ap-_ 
