170 CHORDATA. 



vitreous body) and a fluid, the aqueous humor, and its walls of three con- 

 centric layers. The outer of these is the tough protecting sclera (sclerotic), 

 a usually fibrous, but in many fishes a cartilaginous, layer, which in front 

 is transparent and strongly curved, forming the cornea. The second layer, 

 the chorioid coat, is richly vascular and pigmented; at the boundary be- 

 tween sclerotic and cornea it is changed .to the iris. The inner layer is the 

 retina, the structure and arrangement of which are characteristic of the 

 vertebrates. 



From the developmental standpoint the retina (fig. 85) consists of two parts, 

 the retina proper and the tapclnm nigrum (pigmented epithelium), formerly 

 regarded as part of the chorioid. In the retina the following layers are dis- 

 tinguished: (i) the limitans interna; (2) nerve-fibre layer; (3) ganglionic layer; 

 (4) inner molecular layer; (5) inner granular layer; (6) outer molecular layer; 

 (7) outer granular layer; (8) limitans externa; and (9) layer of rods and cones. 

 The limitans externa is the bounding membrane of the embryonic retina, which 

 is later penetrated by the rods and cones. Between the two limiting membranes 

 Mailer's fibres ('m) extend, large supporting cells, the nuclei of which lie in the 

 inner granular layer, and which are aided in their supporting function by the 

 fine horny framework of both molecular layers. The nervous elements which 

 are imbedded in this support are best understood by beginning with the optic 

 nerve. This spreads out in the nerve-fibre layer, and on its way to the end 

 apparatus comes twice into relation with ganglion cells; first in the ganglionic 

 layer, second in the inner granular layer, the 'granules' being largely the nuclei 

 of bipolar ganglion cells. Thus a great part of the retina (layers i to 6) are to 

 be considered as an optic ganglion, such as occurs in molluscs and arthropods, 

 but which there lies outside the sensory apparatus. The sensory epithelium 

 (the retina in the sense this term is used in invertebrates) consists of but two 

 layers, the outer granular layer and the rods and cones. The outer granules are 

 the nuclei of the extremely slender epithelial cells which bear the rhabdomes 

 (rods and cones) on their peripheral ends. The pigment so necessary for the 

 visual function is supplied by the tapetum nigrum already mentioned. This is 

 a layer of hexagonal epithelial cells which lies on the tips of the rhabdomes and 

 sends pseudopodia-like processes between them, and since it is rich in black 

 pigment granules, the rods and cones are enveloped in a close pig merit mantle. 



< 



Although in this relation of pigment and in the union of the optic 

 ganglion with the sensory cells important differences are to be noted from 

 the eyes of the invertebrates, even from the closely similar cephalopod eye 

 (P- 337)j the most striking difference remains to be mentioned. The 

 retina abuts with its limitans interna and nerve-fibre layer against the 

 vitreous body; with its rhabdomes and tapetum against the chorioid. 

 Hence the incoming light must traverse the optic ganglion and pass 

 through the layer of sense cells before reaching the end organs, the rhab- 

 domes. In nearly all invertebrates, for example the Cephalopoda (fig. 

 349), the light falls directly on the peripheral end of the rhabdome. The 

 rhabdomes in cephalopods, as in most invertebrates, are turned towards 

 the light, in the vertebrates away from it. 



