M. T. Thorell on the Terminology of the Arguliclse. 449 



Neither am I acquainted with any transition between the Pajci- 

 lostonia and Siphonostoraa. It is freely granted that, in such 

 forms as lack the appendages of the mouth, it may be sometimes 

 difficult to determine to which series they should be referred; 

 but in such instances correspondences in other parts of the ge- 

 neral structure must decide the question : for example, it is 

 easy to see that Monstrilla is a Poecilostome and approaches the 

 Corycseidse, to which family it is, indeed, referred by Claus. 



From what has now been said, it follows that I cannot admit 

 that my arrangement of the Copepoda renders difficult the forma- 

 tion of subordinate groups, families, and genera. On the other 

 hand, it is conceded that it sometimes removes from each other 

 forms which in habitus stand tolerably near together; but this 

 inconvenience is in a great measure compensated by the paral- 

 lelism of the series. Certain it is that by the division of the 

 Copepoda into Copepoda carcinoidea and C. parasitica, adopted 

 by Claus, this inconvenience is not avoided : any definite limita- 

 tion of these two groups based upon characters drawn from the 

 form is not to be thought of. Claus himself admits this, but 

 consoles himself with the consideration that the impossibility of 

 a sharp definition of limits lies in the very nature of any system 

 which would be true to nature. Many, however, will be found 

 who will agree with me in not resting content with such reason- 

 ing, but in regarding fixed principles for the forming of divisions 

 as necessary for any systematic arrangement. And if the source 

 of such division be sought iji the modifications of organs which 

 are constant in their nature and significance throughout the 

 entire life of the animal, which has seemed to me to be the case 

 with the oral organs, a sharp definition of the limits of groups 

 will not necessarily make the system one-sided and unnatural. 



\_Note. In a rather lengthy footnote appended to the preceding 

 paper. Prof. Thorell makes the following important remarks re- 

 lative to the nomenclature of the various portions of the body 

 in the Argulids. Reverting to p. 150, we find that Prof. Thorell 

 applies the terms headov head-shield, trunk, and tail to the prin- 

 cipal divisions of the body in Argulids, calling the pieces at- 

 tached posteriorly to the latter appendages, not postabdumen. In 

 connexion with this he says : — 



" Such a terminology differs somewhat from the now generally 

 received division of the Crustacean body into head, thorax, abdo- 

 men, and pustabdomen. There are several objections to this divi- 

 sion. Thorax and abdomen are divisions founded (throughout 

 the greater portion of the Articulate series) on notions almost 

 exclusively drawn from the class of Insects, and are not properly 

 applicable to any but that class and the Arachnids, where they 



