676 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxv. 



a cephalo-thovax, a free thorax, and an abdomen — the first and third 

 being unsegmented, while the free thorax is divided into three well- 

 defined segments. These regions thus correspond closeh^ to those in 

 the Eucopepods. The anterior one has always been called the cephalo- 

 thorax, but there has been considerable question as to how many 

 segments of the thorax were fused with the head. 



The answer to this question, and with it the solution of the whole 

 problem of the body regions, the other two of which have received 

 all the different names possible, rests entirely upon our interpretation 

 of the two pairs of appendages which, in common with Claus and 

 Thorell, we have designated as the anterior and posterior maxillipeds, 

 but which are really the endopodite and exopodite of the posterior 

 maxilla?, separated, and each joined to the body by an independent 

 basipodite. If this latter statement be true, then of necessity only a 

 single segment of the thorax is fused with the head. The free seg- 

 ments following must be the remainder of the thorax, and the unseg- 

 mented posterior portion is the abdomen. The only other supposition 

 possible is that the appendages in question are anterior thoracic legs, 

 which has ])een earnestly advocated by several authors. 



If we can decide between these two alternatives, all the other prob- 

 lems will solve themselves. 



First, then, the evidence in favor of regarding these appendages as 

 anterior thoracic legs rests entirely upon their present form and func- 

 tion. But the value of such evidence must disappear the moment we 

 reflect that these animals are parasites, for we have every reason to 

 believe that both the form and the function of many appendages 

 would of necessity be radically changed by long-continued parasitic 

 habits. 



In proof of this, witness the present form and function of the antennte 

 and mouth parts. Why may there not have been a similar change in 

 the posterior maxilUv ? 



Again, the first pair of tiiese ap]3endages is placed so far forward 

 that, if they are really thoracic legs, the mouth parts are entirel}^ out 

 of place for a crustacean, and particular!}' for one so closely related to 

 the copepods. 



Furthermore, if w^e are to look upon them as grasping or Avalking 

 legs, it is evident that the appendages intervening between them and 

 the mouth (i. e., the posterior maxilhe and the maxillipeds) have been 

 suppressed, and that, too, so thoroughly that not a trace of them is left, 

 even in early development. 



But the appendages which do remain are perfectly developed, and 

 such a complete suppression without assignable cause demands far 

 better proof than mere present form and function. 



The best argument, however, is derived from analogy. These two 

 pairs of appendages correspond almost exactly in form, position, and 



