238 ADAPTATIONS TO NOCTURNAL ACTIVITY 



with which to cover it up — all of the fuscin being concentrated in small 

 masses in the tips of the pigment-cell processes, as in the sturgeons (v.i.). 



A retinal (guanin?) tapetum is common in bathypelagic teleosts; and 

 it may be occlusible in the young, which characteristically live much less 

 deeply than the adults, and have both reflective material and fuscin in 

 their pigment cells. The fuscin disappears during growth, so that the 

 adult tapetum is certainly fixed. Like the argentea, the tapetum is lacking 

 in bathybic teleosts which never come near the surface. 



The guanin tapetum formed in the pigment epithelium of the croco- 

 diles and their allies is non-occlusible, for the cells contain much guanin 

 and only a little fuscin, which migrates but feebly and is inadequate to 

 blanket the guanin from the light. If we assume that the guanin was put 

 there early in the evolution of the group, before the photomechanical 

 changes dwindled as they have in these reptiles (p. 162), we can imagine 

 that the crocodiles once had an occlusible tapetum but found it unneces- 

 sary to maintain it once they had developed an efficient vertical-slit pupil. 



Other Retinal Tapeta — Other non-occlusible retinal tapeta are those 

 of the fruit-bats and that seen in the common opossum, Didelphis vir- 

 giniana (but not in Marmosa, though all opossums have eyeshine). The 

 opossum structure occupies the superior half of the eye-ground, and is a 

 neat semi-circle with its straight margin running horizontally at the 

 level of the disc (Fig. 93c) . The pigment epithelial cells below this level 

 have their normal, dense content of fuscin granules; but in the modified 

 area (Fig. 95) they are twice as tall, devoid of pigment, and packed full 

 of microscopic particles which look like guanin but apparently are not. 

 These granules dissolve readily in histological reagents which guanin 

 resists, and are hence not seen in micrological sections of opossum eyes. 

 The pale yellow particles with which the pigment cells of the fruit-bats 

 are filled are likewise of unknown chemical composition. In the dog, the 

 retinal pigment epithelial cells covering the tapetized part of the chorioid 

 are themselves filled with reagent-resistant reflective particles, which have 

 never yet been accurately studied or described. 



Guanin in Chorioidal Tapeta — Guanin also occurs in chorioidal 

 tapeta. That of the sturgeon bears a superficial resemblance to a tapetum 

 cellulosum, with up to twelve layers of cells; but the cells are filled with 

 guanin or a closely related substance (Fig. 96) . The pigment epithelium 

 has not been able to rid itself entirely of pigment in the portion which 

 overlies the tapetum. Instead of there being a little pigment in each cell 



