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

 gradually with the likeness in function. 



We may now pass on to a nearer examination of the grounds 

 on which Kroyer* sought to prove that the Argulidse are sipho- 

 nostomous Copepoda most nearly related to the Caligidse. 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 Argulidse : they 

 neither occur again in Branchiopoda nor in Copepoda; and 

 thus Gyropeltis might as well be said to form a passage to the 

 Apodida, for instance, in the former order as to the Caligidse 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 Argulidse 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 Argulus in the larval 

 condition. 



Another reason for his view of the relation between the Argu- 

 lidse and the Caligidse has been drawn by Kroyer from the 

 structure of the antenna : he endeavours, in fact, to identify the 

 first pair of antenuse 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 Argulidse demonstrate, in the fact that 

 in them not the second, but the first pair of antennse have be- 

 come the fixing-organs, that they are widely separated from the 

 Caligidse and the other parasitic Copepoda. Kroyer further tries 

 to show that Argulus, like the Caligidse, has two pairs of foot- 



* Loc. cit. pp. 25-29. I have not been able to see the force of the 

 following objection aj^ainst Zenker's separation of the Argulidse from the 

 Siphonostonia : — " 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 li Argulus 

 may not be united with the Siphonostonia, neither may it be enrqlled 

 amongst the Branchiopoda; else why think of erecting a new order for 

 this genus?" 



