476 TRANSACTIONS OF THE 



The eyes of the hassar have a distinct cornea extension to the 

 iris, but only of very slight convexity. This appears to be the 

 case with all fish and animals inhabiting water. The difference 

 of density between the external medium and the cornea is incon- 

 siderable, as the refractive power of the cornea, however convex 

 it may happen to be, is small. The crystaline lens performs the 

 necessary refraction of rays. The cornea is, therefore, nearly 

 always perfectly flat, and the globe of the eye in the shape of a 

 hemisphere; while the lens is spherical. The sclerotic coat of 

 the eye is of great deitsity and thickness, and aids it in retaining 

 its form. The eye of the shark is supported at the bottom of 

 its orbit by a cartilaginous point that affords it the facility of 

 turning on a pivot. His visual organs are infested by three 

 varieties of eye- worms, which sometimes interfere with that dis- 

 tinct sight for which this fish is celebrated. When these animal- 

 culse increase in number to about three hundred in both eyes, 

 they cause cataracts, and the fish becomes blind. This worm 

 resembles the leech, except that its sucking apparatus is in the 

 middle. And what is singular, each of these creatures has a 

 parasite to support which belongs to the genus Monas, and is 

 much smaller than the Atomus. All fish are infested with these 

 worms. 



And not their eyes alone, but their gills likewise. These 

 latter appear to be compound animalculse, having one body but 

 many moutlis, each containing precisely the same organs. Their 

 fceces, as in the polypes, pass out at their mouths. Last week I 

 observed a perch taken from the river, that appeared to be 

 inactive and indisposed to move. On examining him, I found 

 attached to his tongue a fresh-water branchipod, which could not 

 be taken out without force sufficient to break the arms attached 

 to his sucker. On removing him, I found a small insect clinging 

 to his abdomen, which likewise lost its arms on being detached. 

 Its mouth consisted of a tongue, upper lip, two mandibles, two 

 pair of maxilla, and it had sixty legs. One eye of the perch was 

 covered with a cataract, and the other partially so; in each there 

 were numerous planaria, presenting the appearance of a cross, 

 each arm of which was possessed of an alimentary canal, circu- 



