278 M.T. Thorell on the Systematic Position of the Argulide. 
entire organization. Leydig appears to be the first who recog- 
nized in the tail-fin of the Argulide their principal and proper 
respiratory organ, at the same time admitting that, as in the 
lower Crustacea in general, respiration is here also effected 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 Pandaride. 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 Argulide 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 lke the swimming-feet of the Argulide. 
In order, however, not to overlook this likeness, we must not 
select for comparison 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 Argulide, together with the swimming-branches and “ fla- 
gella”’ attached to the end of this. The sole difference is that 
this “ flagellum” in the Argulidz is bent upwards and inwards ; 
but even this is not always the case; for in A. funduli it is, 
according to Kréyer, 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- 
gulide: 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 
* Loe. cit. p. 338. 
