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



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 

 Argulidse belong. 



As we have already shown, Kroyer has mistaken these parts 

 so far as to regard the second pair of antennae 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 Argulidse and the genital ring in the Caligida?, 

 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- 

 lidse corresponds to the entire tail, inclusive of the genital ring, 

 in the Caligidse and other Copepoda. This, however, is alto- 

 gether foreign to the question how far the Argulidse 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 Argulidse 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 Argulidse 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 Crustaces 

 suqeurs, "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 bfver tvenne markliga Crustaceer of ord- 

 ningen Cladocera," CEfv. af. K. Vetensk. Ak. lorh. 1860, p. 266. 



