572 HIGHER FISHES 



fuscin pigment in the retinal layers is as in elasmobranchs (p. 567) ; 

 whereas, where there is much stromal pigment and little or no guanin, 

 the posteriormost epithelial layer of the iris is quite unpigmented, as 

 in lampreys. 



The Retina — The sturgeon retina is characterized by a peculiar pig- 

 ment epithelium, normal enough in its heavy pigmentation where it 

 covers the 'ciliary body', but modified opposite the entire sensory retina 

 in a manner best understood in connection with the discussion of the 

 tapetum — pp. 238-9. Another peculiarity is the virtual absence of any 

 distinct inner nuclear layer. The neuron cell-bodies which should form 

 such a layer are displaced upward or downward by the great mass of 

 horizontal cells (Fig. 96, p. 242) ; and the Miiller fibers are not evenly 

 distributed, but gathered into great bunches, their nuclei squeezed up to 

 the lower surface of the outer nuclear layer. The outer nuclear layer is 

 essentially single, but ragged, with the cone nuclei lying above the ex- 

 ternal limiting membrane, and the rod nuclei nearly always below it 

 except in the periphery. Summation in the scanty ganglion cells is very 

 great, and the overall threshold of stimulation of the retina should be 

 very low, in keeping with the habits of these fishes. 



The visual cells (Fig. 168, p. 570) are of two types — large rods, and 

 single cones in smaller number. Here, for the first time (phylogenetic- 

 ally), we encounter cone oil-droplets in an extant vertebrate group. The 

 oil-droplets are completely colorless in life (A. fulvescens, at least), but 

 were assuredly not always so. The very fact that so many cones are 

 present — though with their oil-droplets bleached in sympathy with a 

 present avoidance of strong light — together with the presence of an 

 apparently vestigial mechanism for moving the lens (the papilla de- 

 scribed above, which suggests that the ancient chondrosteans did have 

 accommodation), indicates that the primitive chondrosteans were diur- 

 nal, probably with smaller rods, more cones, and an accommodation 

 equal to that of the teleosts. Moreover, though double cones (which are 

 associated with bright-light vision) are lacking in living sturgeons, their 

 presence (and identity of plan) in both the holosteans and the amphib- 

 ians shows that the common ancestors of these groups, the primitive 

 Chondrostei, must have had them (and presumably invented them; see 

 Plate I) . The oil-droplet is probably even more ancient, and indeed may 

 have been present in the visual cells of vertebrates before these were 

 visual in function : such pigmented oil-droplets are common in pigment- 



