617 



order the first segment of the hind body is the genital segment, which is 

 always without legs. In the Podoplea the fiftli segment belongs to the 

 hind body, its legs are always rudimentary and never modified into 

 copulatory organs, so that in this suborder the first segment of the hind 

 body is the fifth, and it, together with the genital segment, carries rudi- 

 mentary legs (Die Asterocheriden, p. 48). Such a diagnostic character 

 may serve very well for tbe free Copepods, but in quite a large pro- 

 portion of the fish parasites there is no body segment carrying a fifth 

 pair of legs either in the adult male or female, or in any stage of their 

 development. Achthei'es and Leniaea are excellent examples from two 

 of the families. 



The adults of both sexes are so degenerate as to give no evidence 

 at all, and this degeneration commences at such an early stage in their 

 life history that we shall never be able to decide with certainty whether 

 the fifth segment belongs with the fore or hind body. Lernaea possesses 

 at one stage of its development four pairs of legs but does not show 

 even the rudiments of a fifth pair, while Achtheres develops only two 

 pairs with the extreme rudiments of a third. Hence, even if we could 

 locate the fifth body segment, there must of necessity be lacking the two 

 pairs of rudimentary legs which characterize the first two segments of 

 the hind body in the Podoplea. 



Again in the Caligidae the arrangement of the muscles in the adult 

 clearly demonstrates that the movable articulation is between the third 

 and fourth thorax segments and not between the fourth and fifth, and in 

 living specimens that is where we find flexion actually taking place. 

 Practically therefore the fourth somite is as firmly connected with the 

 hind body as the fifth , and the first three segments of the hind body 

 each carry a pair of legs in some of the genera, while in others there is 

 only the pair on the fourth segment and not even the rudiments of any 

 on the fifth or the genital segment. 



c. A third objection to Giesbrecht' s system is very clearly stated 

 by Smith. "The adoption of this classification necessitates our separat- 

 ing many families which superficially may seem to resemble one another, 

 e.g., the semiparasitic families Lichomolgidae and Ascidicolidae, and the 

 Dichelestiidae from the other fish-parasites ; italso necessitates our treat- 

 ing the presence of a sucking mouth as of secondary importance" (p. 63). 



The presence or absence of a sipho or beak ought to be of secondary 

 importance according to the views here stated, but not so the separation 

 of closely related families. The resemblance between the Dichelestiidae 

 and some of the other fish parasites, the Caligidae for example, is far 

 more than superficial, and to separate them entirely from all the others, 

 as is here done, is both unscientific and unnatural. 



