222 PISCES. 



may be proved to exist. The Rays, e.g. Torpedo, exhibit a peculiar 

 disposition of the pupillary margin, for golden-colored processes, 

 shaped like palm branches, depend from the iris, and are capable of 

 closing the pupil like a curtain. 



The Optic nerve in Fishes is generally formed of plicated laminae, 

 which can be unfurled from each other like the vanes of a fan. It 

 perforates the sclerotic obliquely, and at a distance from the cen- 

 tral axis of the eye. The point of exit for the retina is a round 

 opening, frequently, however, an elongated slit, from which the 

 nervous tunic of that membrane expands. The latter exhibits the 

 same component parts as in the remaining Vertebrata, such as a 

 strongly developed racemose or papillary layer arranged in a peculiar 

 manner. . 



The Crystalline lens is of large size, perfectly spherical, and en- 

 closed within a thick capsule ; it usually protrudes through the 

 pupil, and approaches, like the iris, very close to the cornea, so that 

 the posterior chamber of the aqueous humor is completely wanting, 

 and the anterior very small, containing only a very slight quantity of 

 that fluid. The lens consists, as in other animals, of peculiar fibres, 

 which are united by toothed edges, like the sutures of the cranial 

 bones. The Vitreous body, flat, and having the lens deeply imbedded 

 in it, contains a small quantity of thin fluid. 



The eye of a Fish, when most perfectly constructed, presents 

 always a very flattened form of bulb and a short axis ; owing to the 

 watery medium in which they live being denser than that of air, the 

 vitreous and aqueous humors appear to be present in smaller 

 quantity, while the lens is more dense and spherical, so as to bring 

 the rays of light to converge at a shorter focus. But besides these 

 general, other special peculiarities occur in the structure of the eye- 

 ball in Fishes. Thus in Pleuronectes both eyes are placed asym- 

 metrically upon one side. In Anableps tetrophthalmos the cornea 

 of each eye is divided by a transverse band, and is thus rendered 

 double along with the pupil, while the other parts remain single. 

 The size to which the eyes are developed varies greatly ; thus they 

 are largest in many Osseous Fishes, and particularly so in the Pla- 

 giostomi, where, as in Priacanthus, they are of remarkable magni- 

 tude, and still more so in the rare Pomatomus telescopium, which 

 lives in the greatest depths of the Mediterranean sea. The eyes are 

 generally small in those Fish that bury themselves, as the Eels and 

 Cyclostomi, in mud and sand. There are Fishes also with very 

 small and rudimentary eyes, e. g. Silurus coecutiens and Apterichthus 



