M. T. Thorell on the Sijstematic Position of the Argulidse. 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 Argulidas, 

 on the contrary, most generally two pairs, both maxilla3 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 antennae of the 

 second pair which are used as seizing-organs ; in the Argulida?, 

 on the contrary, it is the first 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 joints. Ge- 

 nerally the feet belonging to one pair are united by means of a 

 median i^late, 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 

 Caligidse. 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 Argulidse, 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 in 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 laFge, 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 Pontellida) and Corycseidse) show a struc- 

 ture entirely different from the compound eyes of the Branchio- 



