280 M.T.Thorell on the Systematic Position of the Argulide. 
jaws. ‘This we are the less disposed to dispute since we regard 
two pairs of foot-jaws as belonging typically to all the lower 
Crustacean orders; whence it follows that this character leaves 
quite undetermined the question to which of these orders the 
Argulide belong. 
As we have already shown, Kroyer has mistaken these parts 
so far as to regard the second pair of antenne as being the first 
pair of foot-jaws, and the sucking-cups, or the true first pair, as 
the second. Kroyer has further, by drawing a parallel between 
the tail in the Argulidee and the genital ring m the Caligide, 
thought it possible to establish a nearer affinity between these 
families. I have already pointed out the incorrectness of this 
comparison, and endeavoured to show that the tail of the Argu- 
lidee corresponds to the entire tail, inclusive of the genital rmg, 
in the Caligide and other Copepoda. This, however, is alto- 
gether foreign to the question how far the Argulide are Cope- 
poda or Branchiopoda. The Branchiopoda have a jointed or 
unjointed tail like the Copepoda; and in many Phyllopoda (as, 
for instance, Branchipus and Artemia) the first caudal segments 
are much more subservient to the office of generation than in 
the Copepoda and Argulus, since they contain, as already men- 
tioned, both testes and ovaria. Even among the Cladocera, 
which otherwise, with few exceptions (Leptodera, Bythotrephes), 
resemble the Argulide in their unsegmented tail, we have an 
example of this in Leptodera hyalina (of which, however, the 
male is unknown), in which animal the ovaries at least are 
situated in the anterior tail-segments*. 
Kroyer further asserts that, since it is now known that the 
sting ” does not belong to the mouth-tube, the oral organs of 
the Argulide offer no difficulty to their union with the Sipho- 
nostoma. We have, however, shown that the only resemblance 
between the oral organs in the two groups is that the mouth 
forms a sucking-tube, as is the case so often amongst the parasitic 
Articulata, not only among Crustacea of different orders, but 
also among Insects and Arachnids. That this character is of 
extremely subordinate value in the determination of the sys- 
tematic position of the Crustacea is shown by the circumstance, 
among others, that we are forced to unite in the order Copepoda 
forms with free oral organs and forms with these enclosed in a 
suctorial tube. Zenker, who showed the necessity of this, rightly 
observes, with respect to Milne-Edwards’s subclass Crustacés 
suceurs, “Is the form of the mouth to be regarded as of such 
weight systematically in a case where parasitic habits call for 
and produce a certain determinate form dependent thereupon ? 
* Lilljeborg, “ Beskrifning ofver tvenne miarkliga Crustaceer of ord- 
ningen Cladocera,” ify. af. K. Vetensk. Ak. Forh. 1860, p. 266. 
