332 PHILOSOPHY OF ZOOLOGY, 



The sclerotic coat of the eye of fishes is more firm and 

 dense than in the higher animals. It is here cartilaginous, 

 semitransparent, and elastic, and sufficiently solid to pre- 

 serve its form of itself. In the salmon it is of the thickness 

 of a line posteriorly, and of an almost bony hardness before. 

 This is frequently the case in other fishes, especially near 

 its junction with the cornea, where it sometimes appears 

 like an osseous ring. The outer layer of the choroid coat 

 is either white, silvery, or gold-coloured, and is very thin 

 and little vascular. The inner coat, to which the term 

 membrana Ruyschiana has been applied, is in general 

 black, and covered everywhere by mucous substance. In 

 the ray, however, it is transparent. Between these two 

 membranes of the choroid coat there is a body of a bril- 

 liant red colour. Its form is usually that of a thin cylin- 

 der, formed like a ring round the optic nerve : the ring, 

 however, is not complete, a segment of a certain length 

 being always wanting. Sometimes, as in the Perca Idbrax, 

 it consists of two pieces, one on each side of the optic 

 nerve. It is considered by some as muscular, and enabling 

 the eye to accommodate its figure to the distance of the 

 objects ; while others regard it as glandular, and destined 

 to secrete some of the humours of the eye. This gland, 

 we may add, does not exist in the Chondropterygii, as the 

 rays and sharks. 



The iris is in general distinguished by its golden and 

 silvery brilliancy. This arises from its transparency, al- 

 lowing the natural colour of the choroid coat to be dis- 

 cerned. The pupil is different in form in the different 

 species, but, in general, it approaches to circular or oval ; 

 in some genera, as the salmon, it projects into an acute 

 angle at the anterior part. In the Gobitis anableps of 

 LINNAEUS, the cornea is divided into two portions, and 

 there is a double pupil with a single lens. In the ray, 



