470 CHORDATA. 



■\"itreous bod\'~l and a lluid, the aqueous humor, and its walls of three eon- 

 eentrie lavers. The outer of these is the toui^h proteeting sclcrd (.wlirollc), 

 a usualh' tilu'ous, but in mam' tishes a eartilayinous, ht\"er. whielt in front 

 is transparent and stronL;l\" eurxed. formiuL;- tlie conu\i. The seeond layer, 

 the cl:orio!J coal, is riehlv \'asciilar and piLj:niented ; at the boundary be- 

 tween selerotic and eornea it is ehan^'ed to the /;;'.\'. The inner la_\"er is the 

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

 vertebrates. 



From the developmental standpoint the retina ifiL;. 85"! consists of two parts, 

 the rc:ii:d proper ami the laptUnii iii^iniii ^|aL;Inellted e]alheliiiin 1, lornierly 

 rcLjariled as part of tlie chorioid. In the retiiui the follouinL; layers are dis- 

 tin,'uished: i,il the limiiaiis interna; v-' ner\"e-ubre la)"er; vo' i;anulionic la\"er; 

 (4> inner molecular la\'er; ^5! inner granular la\er; (Oi outer molecular layer; 

 (7) outer granular laver; iS) limiians externa; and (ol la\cr of rods and cones. 

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

 is later penelraied bv tlie rods and cones, between the two limiting membranes 

 Mailer's fdjres ('») extend, large supporliuL; cells, the nuclei ol which lie in the 

 inner granular laver, and which are aided in their supporting; fiuiciien by the 

 tine horny frameworlc of both molecular layers. The nerxous elements which 

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

 ner\e. This spreads out in the nerve-Ubre layer, and on its way to the end 

 apparatus comiCS twice into relation with ganglion cells; first in the gaimlionic 

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

 of bipolar ganglion cells. Thus a great part of (he retina (layers i to 0) 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 liear the rliabdonies 

 (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 wdiich lies on the ti]is of the rliabdonies 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 piigment mantle. 



.\lthough in this relation of pigment and iit the union of the optic 

 gairglioir with the sensory cells important dirfereitces are to be itoted from 

 tlie eyes of the invertelirates, e-\en from the closely similar cephalopod eye 

 (p. 037), the most striking difference remaii\s to he mentioned. The 

 retina aliuts with its limitairs iitterna arid itene-libre laver against t.he 

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

 Hence the incoming light must tra\erse the optic gangliotr and pass 

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

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

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

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

 the hght, in the vertelirates away from it. 



