615 



value than the remainder of the mouth-parts, and the latter in turn 

 should outrank the swimming legs. To base the primary divisions of the 

 Copepoda upon the form of one or more of the swimming legs, and to 

 follow this with subordinate divisions derived from the structure of the 

 first antennae, may seem convenient, but it will not prove to be scientific 

 or satisfactory. In proof of this statement see the discussion under 6. 



5) Degeneration may go a step farther and indicate for us which 

 of the appendages in the various groups is likely to possess the higher 

 systematic value. The three nauplius appendages appear simultaneously 

 and development does not enable us to make any selection among them. 

 But in degeneration they disappear one at a time, and the first antenna 

 remains the longest. Consequently the marked differences, for example, 

 in the structure of the first and second antennae ought to outweigh the 

 similarity of the mandibles, in deciding Avhether the two genera Thersitina 

 and Ergasilus are to be fused or kept distinct. Among the other (i. e. 

 beside the mandibles) mouth-parts the maxillipeds disappear first , but 

 the second maxillae are not as easily modified as the first pair. The 

 swimming legs degenerate in exactly the reverse order of their develop- 

 ment, the posterior pair disappearing first. These relative values of the 

 various appendages may well furnish useful suggestions in deciding 

 such systematic questions as the one just referred to. 



6) The parasites and semi-parasites comprise more than 

 three-fifths of the entire group of Copepods. No classification 

 of this group, therefore, can afford to neglect the testimony and the 

 suggestions derived form degeneration. Let us examine the Gies- 

 brecht-Sars system on this basis. We have already stated that Sars 

 adopted Giesbrecht's secondary basis of division; the vital question 

 therefore is with reference to the i^rimary division, Giesbrecht used 

 as the basis of his first division the location of the boundary between 

 the fore and hind body and the structure of the fifth (posterior) pair of 

 swimming legs. 



These are two of the things which possess the very lowest systematic 

 value according to the testimony of degeneration, useful for distin- 

 guishing genera and species, but not stable enough for separating sub- 

 orders and tribes. 



No better proof of the truth of this testimony could be asked for 

 than that which is furnished by the attempted application of such a 

 primary basis in the hands of three as able workers as Giesbrecht, 

 Smith and Caiman. What measure of success has attended their 

 efforts? 



a. Objection has already been made that the resulting system is 

 only a partial and incomplete division of the group. It is worth while 



