806 Description of the Argulus Catostomi. 



they represent. The next pair (representing the first pair of nata- 

 tory legs,) terminate in two branches, one of which is jointed, while 

 the other ends in two setas. The three following pairs of natatories 

 are not developed. Instead thereof are three protuberances on each 

 side of the abdomen, ending each in two setae. 



■ The internal organs of the abdomen as observed are exhibited in 

 fig. 10. 



The larve is quite active, and by means of its oars swims with 

 great agility. Out of a thousand, none lived more than four or five 

 days, probably for the want of appropriate nourishment. During 

 that time they suffered no change. 



From the structure of the mouth as described in the preceding 

 article it is obvious that the name of the order (Siphonostoma) to 

 which the animal must be referred, is not truly applicable, since 

 the siphon is a small part only of the apparatus for manducation. 

 It seems to be a connecting link between the Xyphosura and Sipho- 

 nostoma, and may perhaps hereafter become the type of a new order. 



A plain man, and quite unversed in comparative anatomy, on 

 looking at our Argulus with a lens of moderate power, remarked 

 that it was nothing but a young horse shoe. The animal to which 

 he referred is the Limulus Polyphemus, (commonly called horse shoe 

 or horse foot,^ found so abundantly on our coasts. Although our 

 opinion does not altogether coincide with his, yet we think that be- 

 tween the two, many analogies may be traced. 



In the Limulus the relative sizes of the clypeal and the thoracic 

 segments are in inverse ratio to the same parts in the Argulus ; in the 

 former, the clypeus occupying a large portion, in the latter, but a 

 small portion, of the shell. The prehensile legs of the latter cor- 

 respond in the number and relative size of joints, and in the denta- 

 tion of the haunches, to the posterior pair of manducatory legs* of 

 the former. The semicircular membrane of the former, which is 

 composed of a pair of united legs, represents the tail of the latter. 

 It resembles it in containing near its origin, two seminal pouches and 

 in being furnished with two collateral lamellae at its terminal sinus ; 

 as well as in its general form. The natatory legs of the latter are 

 the analogues of the branchial legs of the former ; their number is 

 however smaller by one, unless we consider the anterior pair as com- 

 posed of two in union, which opinion receives much support from the 

 fact that in the larve this organ is double, 



♦ In this pair the number of joints is six ; in the four preceding pairs, one less. 



