M. T. Thorell on the Systematic Position of the Argulide. 271 
define what is common to both types and what are the principal” 
characters which authorize their position as two distinct orders 
in the class, and then to see in what respect the Argulide assi- 
milate to the one or the other group, or differ from both. We will, 
in passing, merely recall the common bond which unites all the 
lower orders of Crustacea, Xiphura, Branchiopoda, Ostracoda, 
Copepoda, and Cirripedia, and which seems to us to warrant the 
union of these orders into one large subclass, Entomostraca, in 
Latreille’s acceptation of that term, in contradistinction to the 
other, higher Crustacean orders, or Malacostraca. The Xiphura 
incline towards the Phyllopods in the order of the Branchiopoda, 
with which Zenker was disposed to unite them; the near affinity 
of the Ostracoda with the Cladocera is pretty generally recog- 
nized; moreover they are often placed in close connexion with 
the Copepoda ; and these latter show not only a great affinity 
with the Ostracoda, but also with the Cirripedes on the one 
side, and the Branchiopods, especially the Phyllopods, on the 
other. 
This near relation of the Copepods and Phyllopods shows it- 
self especially in their development. In both groups, the larva, 
as far as is known, always goes through a Nauplius-stage, and 
then shows three pairs of extremities of which the first two 
develope into the first and second pairs of antennee, the third 
into the mandible and its palp when such exists. The body is 
always (except in the lower parasitic Copepods) conspicuously 
segmented; the oral organs consist, when complete, of four 
pairs of appendages s, one pair of inaxillee, 
and two pairs of foot-jaws. The feet are cloven or lobed swim- 
ming or respiratory organs. The Cladocera differ in their in- 
distinctly segmented trunk, and in the fact that they go through 
the metamorphoses which correspond to the Nauplius-stage in 
the egg, and thus do not go through a (true) metamorphosis. 
In this respect they approach the Ostracoda. The development 
of the Argulide is midway between that of the Phyllopods (and 
Copepods) on the one side, and that of the Cladocera (and 
Ostracoda) on the other: their larve go through the earlier 
phases of the Nauplius-stage in the egg-shell, and quit this in a 
form which most nearly corresponds with what Claus calls the 
final Nauplius-stage of the Copepoda ; whence the metamorphoses 
of the Argulide, as compared with those of the Phyllopods and 
Copepods, may be called incomplete. For the rest, we find in 
the Argulide also those characters which we have mentioned as 
common to Phyllopods and Copepods. 
If we would determine by what characteristics the Copepods 
and Branchiopods may always and with certainty be distin- 
guished from each other, we cannot at the outset fail to per- 
