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



Ai'gulidse than to any other group of the Crustacea. Even 

 Vogt expresses himself against the union of the Arguhdje with 

 the Siphonostoma, on the ground that the organization of the 

 mouth is very different ; but he says nothing further as to their 

 place in the system. 



The order Branchiopoda only can claim a stronger affinity 

 to the Argulidse on the ground of several leading points of 

 resemblance. The relation of these latter to that order had 

 already attracted the notice of authors who ranged them with 

 the Siphonostoraa, as, for instance, Milne-Edwards, who says, 

 with reference to the swimming-feet of the Argulidse, that they 

 seem to be intermediate between those of the Branchiopoda and 

 of the ordinary Siphonostoraa. Zenker is, however, the first 

 who made a decided step in this direction, and in his Crustacean 

 system united them with the Branchiopoda, at the same time 

 drawing attention to their marked agreement with these in cer- 

 tain important points, as the structure of the eyes and digestive 

 apparatus, and showing also that the greater number of the 

 characters which were thought to justify their amalgamation 

 with the Copepoda are also to be found in many Branchiopoda. 

 This notion of Zenker as to the systematic position of the Argu- 

 lidae has been shared by Gegenbaur and myself"^, as also by 

 Steenstrup and Luetken, who ally Argulus to the Phyllopoda, 

 as the representative of parasitism in that group. 



A renewed examination of the structure of the Argulidse, as 

 compared with that of the Copepoda and the Branchiopoda, has 

 further convinced me of the correctness of Zenker's view. I 

 have fancied that I could discover grounds for making the Ar- 

 gulidaj a suborder of the Branchiopoda, consequently a group of 

 equal value with the Phijllopoda and Cladocera, although most 

 nearly allied to the former; and as I shall now proceed to an 

 exposition of those grounds, I have only to hope that such may 

 in some measure contribute to the final solution of the question. 

 Such an examination has appeared to me all the more necessary 

 since Kroyer has lately become a champion of the old view of 

 the position of the Argulidse, and has sought to prove their 

 affinity with the Copepoda by several considerations which have 

 been more or less overlooked by previous authors, who founded 

 their ideas of the propriety of inserting the Argulidse among 

 the parasitic Copepoda exclusively on the suctorial formation of 

 the mouth and the parasitic habits depending thereon. 



In order to form a clear notion of the relations of the Argu- 

 lidse to the Copepoda and Branchiopoda, it is necessary to 

 determine the reciprocal relations of these two groups ; i. e. to 



* " Bidrag till kannedomen otn Krustaceer, som lefva i arter af slagtet 

 Ascidia, L.," K, Vetensk.-Ak. Handl. Bd. iii. No. 8 (1859-1860), p. 14. 



