M. T. Thorell on the Systematic Position of the Argulidse. 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 Argulidse 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 PLyllopods 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 antennse, 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 — one pair of mandibles, one pair of maxillae, 

 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 Argulidse is midway between that of the Phyllopods (and 

 Copepods) on the one side, and that of the Cladocera (and 

 Ostracoda) on the other : their larvse 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 Argulida3, as compared with those of the Phyllopods and 

 Copepods, may be called incomplete. For the rest, we find in 

 the Argulidse also those characters which we have mentioned as 

 common to Phyllopods and Copepods. 



If wc 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- 



