298 Description of the Argulus Catostomi. 



We were at first informed by several persons, that the parasite was 

 found adhering to the outside of the fish when taken from the stream, 

 but our own experience has never furnished a sohtary case where this 

 was the fact. We have universally detected it within the branchial 

 cavities ; usually on the operculum or gill-cover, and not on the sub- 

 stance of the gills. On immersing the fish in a vessel of fresh water, 

 the Arguli desert their habitation, and after swimming about a few 

 moments, often attach themselves to the anterior part of the fish, 

 but never, as we could discover, for the purpose of feeding. Not 

 unfrequently they also attach themselves to the sides of the vessel, 

 and there remain many hours. This parasite has hitherto been dis- 

 covered in the Sucker only : and we therefore call it the Argulus 

 Catostomi, a name which cannot be inappropriate even if the ani- 

 mal should hereafter be found on other fish. The Argulus foliaceus, 

 according to all the accounts we have seen, is never observed on the 

 gills, but always on the exterior of the fish. 



The body is covered for the most part by a shell so transparent 

 that the principal organs below may easily be seen. The shell is 

 nearly circular, somewhat broader transversely, slightly convex, with 

 the clypeus extending a little beyond the general curvature. Pos- 

 teriorly, it is divided into two broad lobes by a deep sinus, which 

 gradually widens from its origin and extends as far as the line of 

 junction between the first and second pair of natatory legs, leaving 

 free the three latter joints of the abdomen. The shell is membra- 

 nous and flexible ; above glabrous. Its color is a light sea-green. 

 The border of the shell and a small spot over each pair of antennae 

 are highly diaphanous. Beneath, the duplicature of the shell forms 

 a wide band around the marginal portions, and leaves open a large 

 reniform area on each side, and also another open spot of an irreg- 

 ularly circular form about the central parts of the wings of the shell. 

 The wide marginal band is thickly set with minute reflexed spines. 



The eyes as viewed from above, present twelve or thirteen dark 

 reddish-brown facets, disposed in two concentric curves on a grayish 

 convex receptacle, surrounded, except on the interior side, by a 

 series of colorless facets. 



The antenna, are situated in front of the eyes. The anterior pair 

 is short and stout, two-jointed; basal joint broader transversely ; — 

 terminal joint nearly at right angles with the first, gradually tapering 

 from a broad base, and ending in a large, brown corneous recurved 



