M. T. Thorell on the Systematic Position of the Argulide. 273 
Here it will be proper to make the two following remarks :— 
In the parasitic suctorial Copepoda, the tube never contains 
more than one pair of organs, the mandibles; in the Argulide, 
on the contrary, most generally two pairs, both maxille and 
mandibles, and these in form quite unlike the small saw- or 
lancet-like mandibles of the Siphonostoma. 
In the parasitic Copepoda it is always the antenne of the 
second pair which are used as seizing-organs; in the Argulidee, 
on the contrary, it is the firs¢ pair which performs this function. 
As something peculiar to the order Copepoda, the form of 
the limbs of the trunk holds a prominent place. These consist, 
as we know, of a basal piece, composed of two joints, with two 
branches situated on this, each consisting of three joimts. Ge- 
nerally the feet belonging to one pair are united by means of a 
median plate, so that they move in unison. The structure of the 
legs may perhaps be simplified by certain parts becoming fused 
together or further separated; but whenever the legs attain a 
full development, they present the form here described. Such 
is also the case with the parasitic forms—as, for instance, the 
Caligide. In the Branchiopoda the structure of the legs is 
highly variable, but is still referable to a common type: it is 
sufficient to notice here that they never have the form charac- 
teristic of Copepoda, and that the median plate is always absent. 
This is also the case in the Argulide, whose swimming-feet, as 
we shall show further on, admit of an easy comparison with the 
feet in Apus. The presence of “ branchial appendages” on the 
feet of the Branchiopoda is not a thoroughly constant character 
in this order: they are wholly rudimentary or completely want- 
ing m many Cladocera, as Polyphemus, Podon, Evadne, Bytho- 
trephes, Leptodera, on the extremity of the tail in Nebalia: con- 
sequently the absence of such appendages in an animal (as 
in Argulus) does not show that it cannot belong to the Bran- 
chiopoda. 
Almost as distinctive of the Branchiopoda as the form of the 
feet in the Copepoda is the structure of the visual organs in the 
former as compared with the latter. The Branchiopods have 
two large, generally moveable, lateral eyes (in the Cladocera 
these coalesce into a single eye), composed of numerous crystal- 
line cones, with the cornea at least externally unfacetted. No 
Copepod has eyes that can be fully compared to these; for the 
median single eye in the Copepod, which often contains two or 
more crystalline bodies, corresponds to the ocellus situated be- 
hind the lateral eyes in the Branchiopoda, and sometimes also 
provided with crystalline bodies ; and the large paired eyes in cer- 
tain Copepods (as the Pontellidee and Coryezidee) show a struc- 
ture entirely different from the compound eyes of the Branchio- 
