M. T. Thorell on the Systematic Position of the Argulide. 279 
in Apus; but as the swimming-branches become gradually 
shorter and broader, while the branchial appendage proceeds in 
development, the likeness in form which the first pair of feet so 
strikingly shows to the swimming-feet of the Argulidee decreases 
gradually with the likeness in function. 
We may now pass on to a nearer examination of the grounds 
on which Kréyer* sought to prove that the Argulidz are sipho- 
nostomous Copepoda most nearly related to the Caligide. His 
first argument is that Gyropeltis presents a transition between 
these two families in “certain essential particulars ;” by which 
is meant that the species of that genus lack a sting, and have 
the first pair of foot-jaws of a hooked form, not developed into 
sucking-cups. To this it may be objected that the “sting” and 
sucking-cups are altogether peculiar to the Argulide: they 
neither occur again in Branchiopoda nor in Copepoda; and 
thus Gyropeltis might as well be said to form a passage to the 
Apodide, for instance, in the former order as to the Caligide in 
the latter, on account of its wanting these structures. Through 
the discovery of Gyropeltis, only two of the characters on which 
it would be possible to base the position of the Argulidz as a 
separate order have lost their importance, inasmuch as they 
cease to be constant; but no reason whatever for the union of 
these animals with either of the two orders in question has hence 
been obtained. With respect moreover to the distinct form of 
the first pair of foot-jaws, these organs in Gyropeltis have only 
remained at that stage which belongs to Argu/us in the larval 
condition. 
Another reason for his view of the relation between the Argu- 
lide and the Caligide has been drawn by Kroyer from the 
structure of the antenne: he endeavours, in fact, to identify the 
first pair of antenne in the former with the second pair in the 
latter, on the ground of their functional agreement as fixing- 
organs. We have already had occasion to point out, on the 
ground of their development, the incorrectness of this view of 
their relations ; and the Argulidz demonstrate, in the fact that 
in them not the second, but the first pair of antenne have be- 
come the fixing-organs, that they are widely separated from the 
Caligide and the other parasitic Copepoda. Kréyer further tries 
to show that Argulus, like the Caligidee, has two pairs of foot- 
* Loc. cit. pp. 25-29. I have not been able to see the force of the 
following objection against Zenker’s separation of the Argulide from the 
Siphonostoma :—“ Zenker expresses as the result of his researches, that 
Argulus must either form a separate order or be united with the Branchio- 
poda. But, in admitting this alternative, he necessarily allows that if Argulus 
may not be united with the Siphonostoma, neither may it be enrolled 
amongst the Branchiopoda; else why think of erecting a new order for 
this genus?” 
