1036 CRUSTACEA. 



to call it a pair of true legs. Its form is generally that of a pair of 

 legs, being either elongate, pediform, or furnished with a monodactyle 

 hand, and it differs as widely from proper maxillipeds as the anterior 

 legs in Astacus, or the prehensile legs in Tanais. They are situated 

 in°some genera very close to the mouth, but not so in others; and in 

 none nearer than the anterior pair in Tanais. This pair is normally 

 the seventh in the body, while the first pair in Tanais and all Choris- 

 topods is the eighth; but this is no good objection, certainly no better 

 than could be urged against calling the two anterior pairs in Isopods 

 feet, these pair corresponding to pairs of mouth organs in the Deca- 

 pods. The typical form in the Decapods is based on the existence of 

 but five pairs of legs, the nine more anterior pairs of organs being 

 devoted to the senses and mouth ; in the Choristopods the senses and 

 mouth have but seven of these pairs of organs; and in the highest 

 Entomostracan type, the senses and mouth have but six. 



The legs referred to, that is, the first pair of organs preceding the 

 natatories, are a large stout pair in Caligus and Argulus, in which 

 genera they are called legs. 



Natatory legs. — The natatory legs do not differ essentially from 

 these organs in most other Entomostraca. They are furnished with 

 setse for natation, excepting sometimes the first pair, which, in one 

 or two genera has lateral play, and is imperfectly prehensile. They 

 are always two-branched, and have a two-jointed base, excepting 

 when one of the joints is obsolete. These branches are two- or three- 

 jointed; rarely one of the branches consists only of a single joint. 



There are four pairs of natatory legs of the kind here described, 

 and sometimes also, posterior to these, a fifth pair. But this fifth pair 

 when not rudimentary, has usually a prehensile or subcheliform cha- 

 racter, fitting it for use in coition. Figures 93, 94, and 95 are 

 examples of the rudimentary fifth pair. In figure 94, the leg 

 is reduced to a single joint. In figures 96 to 100, several cheliferous 

 forms are exhibited. Figures 96 and 97 represent each a pair, 

 having the left leg rudimentary, and the right cheliform. Figures 98 

 and 99 are the right leg alone of other species. Figure 100 is the 

 pair of still another genus ; the left is much elongated and subulate, 

 while the right is also long, and besides is subcheliform. Figure 95 

 is female to 96, and 93 is female in the same genus with 100. 



The anterior pair of natatories is sometimes destitute nearly of 



