374 M. T. Thorell on the Systematic Position of the Argulidae. 



poda, if indeed they morphologically correspond with these. 

 The eyes in Argulus are moveable, and agree in their structure, 

 as is well known, with those of the Phyilopods in their most 

 sharply defined form. 



In the Copepoda we never meet with the shell armature vf\\\c\i, 

 as a kind of supplementary integument on the anterior divisions 

 of the body, in the Branchiopoda so generally encloses or covers 

 the body. The large shield in the Argulidse shows the greatest 

 resemblance to the carapace in Apus, and removes them far 

 from the Copepoda. 



With regard to the internal structure, this in many instances 

 affords a more uncertain criterion for judging of the affinity of 

 the lower Crustaceans than do the characters which are drawn 

 from their external form. Thus the nervous system, the circu- 

 latory apparatus, &c., in nearly related forms, may show very 

 striking discrepancies, to adduce an instance of which we will 

 only mention the known fact that, even in the strictly natural 

 group formed by the gnathostomous Copepods, the Calanidse 

 and PontellidsB possess a heart, while the nearly allied Cyclo- 

 pidse, with others, want a definite central organ of circulation. 

 At the same time, however, that the internal structure presents 

 such extraordinary variation and inconstancy even in the most 

 nearly related groups, it shows, on the other hand, in its general 

 features such striking correspondences in the Ei-anchiopods and 

 Copepods, that it is difficult to fix any anatomical character as 

 belonging to one only of these two orders. The generative ap- 

 paratus in the Copepoda only furnishes two peculiarities which 

 are worth noticing here. The eggs are united in the Copepoda, 

 when they leave the ovaries, to form one or two external egg-sacs, 

 excepting in the spurious parasitic Notodelphyidse and Jiupro- 

 ridffi, in which they are received into a matrix, almost as in the 

 Cladocera. In the Branchiopoda such egg-sacs are never pre- 

 sent. In the Copepoda, impregnation takes place, as far as we 

 know, always by means of spermatophores, wiiich are attached 

 externally to the body of the female close to the sexual orifice 

 or the mouth of a separate canal leading to the receptacle. The 

 Branchiopoda never show such spermatophores. In both these 

 points the Argulidse depart from the Copepoda in general, and 

 from the analogous forms in that order, Caligidse and Ascomy- 

 zontidse in particular, in that they lack both egg-sacs and ex- 

 ternal spermatophores. 



Notwithstanding the agreement which the Argulidse present 

 with the Branchiopoda in respect to conditions of propagation, 

 they yet exhibit in this very direction both peculiarities and 

 analogies with other orders. Zenker has already pointed out 

 this : in respect to the form and products of its generative ap- 



