594 SIGHT. 



The eyes of Crustacea are compound and immoveable except in the higher 

 orders, in which they are placed on peduncles and moved by muscles ; and one 

 crab has the peduncle jointed at one spot, so that the eye can be moved in 

 various directions, like the arms of a telegraph. In those Crustacea which are 

 called monoculi, the eye is of the fourth class, many lenses behind one cornea, 

 and placed in a socket, in which muscles exist for its motion. Allusion has 

 been already made to the eyes of mollusca. When they can be useful they exist, 

 but are always simple and solitary ; they have always a lens, and approach very 

 closely to the eyes of fishes and higher vertebrated classes. The sepia?, indeed, 

 besides a large and very convex lens, exceedingly hard at its centre, have a 

 hemispherical vitreous humour, a chorioid, an iris with a kidney-shaped pupil, but 

 no cornea, the integuments of the head being continued over the iris, reflected upon 

 the edges of its pupil, and covering the external surface of the lens Fishes 

 have eyes similar to the sepia?, but possess a cornea, which, as in all aquatic 

 animals, on account of the small difference there can be between its refractive 

 power and the water they inhabit, however great its convexity, is nearly flat. 

 They require no aqueous humour, except a little to preserve the iris free. The 

 vitreous is not a body of sufficient density to be very important. The globe is 

 hemispherical, the lens nearly spherical and very dense, and more and more so 

 towards its centre. It consists of concentric layers of fibres, the fibres of each 

 layer being serrated and locked into another side by side. The pupil is 

 large, and the iris nearly motionless. The outer shining layer of the chorioid 

 passes over the front of the iris and gives it a pearly lustre ; the dark inner 

 layer lines its posterior surface. Their situation renders all eyelids and lachry- 

 mal apparatus superfluous ; and they have only a covering of the common 

 integuments over the cornea. The sclerotic is of extraordinary thickness and 

 hardness, and contains fat between itself and the retina. As fish live in a dark 

 medium their eyes are large, particularly if they live 300 or 400 fathoms below 

 the surface. The eyes of fish which burrow in mud are small j sometimes 

 rudimental or imperceptible. The optic nerves sometimes decussate, sometimes 

 simply lie across each other, sometimes one passes through a hole in the other. 

 Amphibious reptiles, being destined to live on land and in water, have eyes 

 intermediate between those of fish and of land animals. Their eyes are large, 

 possessed of little aqueous humour, and therefore the cornea is comparatively 

 flat, especially in those which are the most in water ; the lens is thick in the 

 direction of the axis of the eye ; as they are sometimes out of the water they 

 have eyelids, the lower larger and more moveable than the upper, together with 

 a third eyelid or membrana nictitans. In land reptiles, the structure recedes 

 from that of the eye of fishes and approaches to what is observed in birds ; the 

 cornea is more convex, the aqueous and vitreous humours more abundant, and 

 the lens less spherical ; two moveable eyelids, a membrana nictitans, straight 

 and oblique muscles, and a lachrymal apparatus exist. In some ophidian rep- 

 tiles, as serpents, the skin of the eyelids passes over the eyes and their appen- 

 dages, and this portion of it is therefore shed with the rest : one small tribe of 

 them, comprehending eels, have a membrana nictitans like saurian reptiles. 

 In some chelonian reptiles, as the tortoise and turtle, and some of the saurians, 

 as the crocodile, a circle of imbricated plates of bone is seen at the fore part of 



