570 



HIGHER FISHES 



cornea is somewhat thinner centrally than peripherally, and is cloudy 

 at its dorsal and ventral margins, as in elasmobranchs. In contrast to the 

 latter, the sturgeons have a prominent Descemet's mesothelium, which 

 is piled up at the iris angle to form an annular thickening from which 

 a loose meshwork bridges over to the iris, isolating a large space com- 

 parable to that of Fontana in mammals (see pp. 679-80) . 



The chorioid is about as thick as the retina (and only a quarter as 

 thick as the sclera), except for a small area in the fundus where a 

 suprachorioidea of richly vascular character (as in some elasmobranchs) 

 is superimposed upon it externally. To receive this, the sclera is locally 



Fig. 167 — Eye of a large sturgeon, Acipenser sturio. xlVi. 

 From Franz, after Soemmerring. co- conjunctiva; sc- scleral 

 cartilage; ct- connective tissue; on- optic nerve. 



Fig. 168 

 Cone and rod from retina 

 of a sturgeon, Acipenser 

 fulvescens. xlOOO. 



thinned, not evaginated. The chorioid is silver-plated inside and out. 

 The inner two-fifths of its thickness is occupied by the laminated, cellular, 

 guanin-laden tapetum lucidum (Fig. 96, p. 242), which is separated from 

 the retinal pigment epithelium only by the choriocapillaris, and extends 

 forward even beyond the ora terminalis to dwindle away opposite the rim 

 of the scleral cartilage. Externally, the chorioid is covered by a (thinner) 

 layer of guaninized tissue — a true argentea, exactly comparable with 

 that of Amia and the teleosts (and presumably directly ancestral there- 

 to) . It is as if either the tapetum or the argentea had been evolved first 

 in the sturgeons, and the other of the two created by delamination, with 



