The form of the _bulbus_ (Fig. 48) is subhemispherical, the cornea (_co_) being flat. If it were convex, as in higher Vertebrates, it would be more liable to injury; but being level with the side of the head the chances of injury by friction are diminished. The _sclerotica_ (_sc_) is cartilaginous in Chondropterygians and Acipensers, fibrous and of varying thickness in Teleosteans, in the majority of which it is supported by a pair of cartilaginous or ossified hemispheroid cups (_c_). In a few fishes, as in _Ceratodus_, _Xiphias_, the cups are confluent into one cup, which possesses a foramen behind to allow the passage of the optic nerve (_o_). The _cornea_ of _Anableps_ shows an unique peculiarity. It is crossed by a dark horizontal stripe of the conjunctiva, dividing it into an upper and lower portion; also the iris is perforated by two pupils. This fish is observed to swim frequently with half of its head out of the water, and it is a fact that it can see out of the water as well as in it. The membranes situated between the sclerotica and retina are collectively called _choroidea_, and 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 argentea_ (_a_), and composed of microscopical crystals reflecting a silvery or sometimes golden lustre. The middle layer is the _membrana vasculosa_ s. _halleri_ (_v_), the chief seat of the ramifications of the choroid vessels; the innermost layer is the _membrana ruyscheana_ or _uvea_ (_u_), which is composed of hexagonal pigment-cells, usually of a deep brown or black colour. In many _Teleostei_ a _rete mirabile_ surrounds the entry of the optic nerve; it is situated between the membrana argentea 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 without pseudobranchia lack a choroid gland. In the Palæichthyes, 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 _pupil_ is generally round, sometimes horizontally or vertically elliptical, sometimes fringed. In the Rays and Pleuronectidæ a lobe descends from the upper margin of the pupil, and the outer integument overlying this lobe is coloured and non-transparent; a structure evidently preventing light from entering the eye from above. In most _Teleostei_ a fold of the _Choroidea_, called the _Processus falciformis_ (_f_), extends from the vicinity of the entrance of the optic nerve to the lens. It seems to be constantly absent in Ganoids.