278 M. T. Thorell on the Systematic Position of the Argulidse. 



entire organization. Leydig appears to be the first who recog- 

 nized in the tail-fin of the Argulidse their principal and proper 

 respiratory organ, at the same time admitting that, as in the 

 lower Crustacea in general, respiration is here also efiected in a 

 great measure by the thin integument of the body, especially 

 on the large membranous head-shield *. Of some use for re- 

 spiration are perhaps also the small leaf-like appendages which 

 are sometimes developed (as in A. purpureus, and, according to 

 Kroyer, in Gyropeltis longicauda) on the final segment of the 

 trunk, and which seem to be analogous to the leaf-like dorsal 

 plates in the Pandaridse. The posterior surface of the last pair 

 of legs is often more or less flattened out and widened ; but this 

 widening cannot functionally or morphologically be compared 

 with the so-called branchial appendages on the extremities of 

 the Branchiopoda. For the rest, the legs of the Argulidse lack 

 any trace of such appendages, but in other respects do not 

 differ much from those of the Branchiopoda; and as it is with 

 the Phyllopoda amongst these that they most agree in general 

 habit, so it is accordingly the extremities of these which are 

 most like the swimming-feet of the Argulidse. 



In order, however, not to overlook this likeness, we must not 

 select for coniparison such extremities as, by their conversion 

 into " respiratory feet,^' have lost the typical form of organs of 

 locomotion, but such as still present this form fully and com- 

 pletely. This is especially the case with the first pair of feet, 

 or the so-called swimming-feet, in Apus. They consist (in Apus 

 cancriformis) of a three-jointed prolonged basal part, which at 

 its extremity bears three long, inconspicuously jointed swimming- 

 branches or flagella). These parts evidently correspond both 

 in form and function to the three-jointed stem of the feet in 

 the Argulidse, together with the swimming-branches and "fla- 

 gella" attached to the end of this. The sole difference is that 

 this "flagellum" in the Argulidse is bent upwards and inwards; 

 but even this is not always the case; for in A.funduli it is, 

 according to Kroyer, directed similarly to the two swimming- 

 branches. Sometimes the flagellum is wanting, as in A. purpureus. 

 The two branchial appendages on the upper surface of the foot 

 in Apus, as also a pair of flagelliform appendages situated on 

 their lower surface, together with the so-called masticatory 

 piece at their base, are, however, entirely wanting in the Ar- 

 gulidse : the supposition that the flagellum in these corresponds 

 to the branchial appendage of the Branchiopoda is con- 

 sequently incorrect — a point which not only its attachment 

 but also its form evidently show. Moreover the parts just 

 mentioned are easily recognizable on the following pairs of feet 

 * hoc, cit. p. 338, 



