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



(2) Palpi are wanting ; 



(3) A " sting " (gadd) is present in front of the mouth-tube ; 



(4) Two pairs of oral organs may be enclosed in this tube ; 



(5) The first pair of foot-jaws may have the form of sucking- 

 cups. 



V. The Argulidae approach the Branchiopoda in general in 



(1) The fundamental form of their extremities; 



(2) The structure of the visual organs ; 



(3) The tendency to a shell-structure, expressed in the shape 

 of the head-shield ; 



(4) The absence of palpi ; 



(5) The absence of external egg-sacs; and 



(6) The absence of external spermatophores. 



VI. To the Phyllopoda in particular they make an approach in 



(1) The conspicuous segmentation of the trunk; 



(2) The non-fusion of the symmetrical eyes into a single eye. 



VII. To the Cladocera in particular they make an approach in 



(1) The small number of the extremities ; and 



(2) The unsegmented tail. 



VIII. They stand midway between the Phyllopoda and the 

 Cladocera by virtue of 



(1) Their mode of development. 



IX. From both Phyllopoda and Cladocera they differ in the 

 following points : — 



(1) They are organized for a parasitic existence ; 



(2) The extremities are entirely without branchial appendages; 



(3) The tail is ti*ansformed into a respiratory plate ; 



(4) The generative apparatus shows some peculiarities, parti- 

 cularly the presence of receptacula seminis ; 



(5) The eggs are ejected immediately after quitting the 

 ovary ; 



(6) Parthenogenesis seems never to occur. 



A glance over this sketch of the relations of the Argulidae to 

 the Copepoda on the one side, and to the Branchiopoda on the 

 other, will, we think, lead to the conviction that the Argulidse 

 cannot possibly be referred to the first-mentioned order, but 

 that, on the contrary, they approach the Branchiopoda through 

 such important characters that their reception amongst these 

 must be regarded as fully warrantable. In our estimation, the 

 Argulidae are parasitic Branchiopoda; and consequently we 

 cannot share the view that they ought to constitute a separate 

 order, in spite of the many points in which, as we have shown 

 above, they differ from both Phyllopoda and Cladocera. Thus 

 the characters which are both necessary and sufficient for de- 



