114 FISHES. 



three in number. The one in immediate contact with the 

 sclerotic, and continued upon the iris, is by no means constantly 

 present; it is the membrana argcntea (ci), and composed of 

 microscopical crystals reflecting a silvery or sometimes golden 

 lustre. The middle layer is the memhrana vasculosa s. halleri 

 (v), the chief seat of the ramifications of the choroid vessels ; 

 the innermost layer is the mcmhra7ia ruysclieana or uvea (u), 

 which is composed of hexagonal pigment-cells, usually of a 

 deep brown or black colour. 



In many Teleostei a rete niirahile surrounds the entry of 

 the optic nerve ; it is situated between the membrana argen- 

 tea and vasculosa, and called the choroid gland (ch). It 

 receives its arterial blood from the artery issuing from the 

 pseudobranchia ; the presence of a choroid gland always being 

 combined with that of a pseudobranchia. Teleosteans with- 

 out pseudobranchia lack a choroid gland. In the Palteich- 

 thyes, on the other hand, the pseudobranchia is present and 

 a choroid gland absent. 



The iris (i) is merely the continuation of the choroid 

 membrane; its capability of contracting and expanding is 

 much more limited than in higher Vertebrates. The piipil is 

 generally round, sometimes horizontally or vertically elliptical, 

 sometimes fringed. In the Eays and Pleuronectidoe a lobe 

 descends from the upper margin of the pupil, and the outer 

 integument overlying this lobe is coloured and non-trans- 

 parent; a structure evidently preventing light from entering 

 the eye from above. 



In most Teleostei a fold of the Choroidea, called the Pro- 

 cessus falci/annis (/), extends from the vicinity of the entrance 

 of the optic nerve to the lens. It seems to be constantly 

 absent in Ganoids. 



The retina (r) is the membrane into which the optic nerve 

 penetrates, and in which its terminal filaments are distri- 

 buted. It consists of several layers (Fig. 49). The outermost 



