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quickly available. One of the most common results of parasitism is that 

 an organ ceases to be used , and in consequence first loses its function, 

 then its size and shape, and finally may altogether disappear. A care- 

 ful study of the comparative anatomy of these parasites shows that in 

 them degeneration is almost an exact reversal of development. They 

 plainly demonstrate the truth of the three principles originally proposed 

 by Vogt in 1878. Parasitism exerts its first influence upon the size and 

 external shape of the body. In the second place it works upon the jointed 

 appendages, beginning with the swimming legs and ending with the 

 appendages of the head. And finally the appendages yield to adaptation 

 in an inverse order to that in which they made their appearance during 

 larval development, the first antennae being the last pair to suffer serious 

 modification. From these facts certain conclusions may legitimately be 

 drawn. 



1) Degeneration may be made a valuable factor in determining the 

 classification of parasitic forms. Being the reverse of development it 

 may be used to corroborate the latter on doubtful points, or may furnish 

 timely suggestions where for any reason the life history is not available. 



2) If the body form offers least resistence to parasitism , it will as 

 readily yield to other transforming influences, and its systematic rating 

 should be correspondingly low. To distinguish a sub-group, or even a 

 family like the Lernaeidae, chiefly by the fact that they possess an un- 

 segmented and highly degenerate body, will be sure to result, as was 

 actually the case, in bringing together degenerate forms belonging to 

 several families, which subsequent investigation must sort over and 

 rearrange. But to distinguish the genus Lernaea from the genus Pen- 

 nella^ both of them highly degenerate, or to separate one species of 

 Caligus from another nearly like it, by their general body form, proves 

 to be scientific, logical and satisfactory. 



3) The testimony derived from body form must always give way 

 to that derived from any of the appendages, whenever there is a conflict 

 between the two. Kröyer located the genus Echetus in the family 

 Lernaeidae on the basis of its body form, not having the opportunity 

 to examine its appendages. As soon as the latter were determined it 

 was transferred from this most degenerate of the Copepod families to 

 one of the least degenerate, the Caligidae, and yet its body form was 

 totally unlike that of any know Caligid. 



4) Among the appendages themselves degeneration emphasizes the 

 testimony of development, that their systematic value varies directly 

 with their appearance during development and indirectly with their dis- 

 appearance during degeneration. 



The three nauplius appendages should possess higher systematic 



