ORGANS OF VISION. 223 



coecus. The eyes are in a still more rudimentary condition in the 

 remarkable, and in many respects anomalous, family of Myxines 

 among the Cyclostomi, which comprises some species that live as 

 parasites upon the internal viscera of Fish, although others are also 

 met with in a free state. The most imperfect state of development 

 is that presented by the Myxine, where the eye is not even visible 

 externally. It consists, in fact, of nothing else than a small bulb, 

 covered by muscles, but which contains a nerve, and internally a 

 transparent nucleus that may be compared to a vitreous body. The 

 rudimentary eye is larger in Bdellostoma, in which it projects some- 

 what above the surface, and is covered by a thin prolongation of 

 integument ; it is enclosed in a spherical cushion of fat, through 

 which passes the delicate optic nerve ; several ocular tunics are 

 distinguishable, which probably contain internally a transparent 

 nucleus. The lowest stage of structure appears, however, to exist 

 in the eye of Amphioxus ; for here we find, as in many of the Inver- 

 tebrate animals, only a couple of pigmentary spots, without, it seems, 

 any organs for intercepting the light in its passage through the eye ; 

 they receive, however, a delicate nervous twig from the cerebral end 

 of the spinal cord. The eyes are also but slightly developed in the 

 Amphibious Fishes, and are devoid of eyelids and muscles ; in the 

 small bulb, however, we may distinguish a cornea and a small lens, 

 while a delicate optic nerve may be also observed. 



In many Fishes peculiar structures are superadded to the organs 

 of vision. To these belong the Choroidal glands of the Osseous 

 Fishes. Between the silver-colored and the internal plate of the 

 choroid there is situated, as already alluded to, in many genera a red 

 vascular mass, which, upon closer examination, is found to consist of 

 a plexus or rete mirabile of vessels, formed by tufted ramifications 

 of arteries as well as veins ; these belong to what is called the am- 

 phicentric system of retia mirabilia, or that with double arterial and 

 double venous vortices, and are connected with the vessels of the 

 accessory branchiae. Those genera which do not possess the latter 

 organs, as Silurus, Pimelodus, Cobitis, and Muraena, have no choroid 

 glands. The choroid, indeed, in all the Vertebrata is properly 

 formed of retia mirabilia, but these are made up by the radiating dis- 

 tribution of arteries and veins from a singlu centre. In the Carp this 

 vascular gland is much developed. 



A still greater peculiarity occurs in many Osseous Fishes, as the 

 Pike, and in the Sturgeon among the Cartilaginous. In all these 

 Fishes, at the point where the optic nerve enters the longitudinal 



