244 



ADAPTATIONS TO NOCTURNAL ACTIVITY 



and nocturnality come and go in evolution as mutatory capacity and 

 ecological expedient direct. Each return to diurnality in a given line of 

 descent will abolish any pre-existing tapetum. Upon a subsequent swing 

 toward nocturnality, the group starts from scratch and is as likely to 

 hit upon one device as another, where so many solutions to the same 

 problem are possible. In no other way can we account for the spotty 



visual cells here 



visual cells here 



Fig. 97 — The occlusible chorioidal guanin tapetum of the elasmobranch fishes. 

 Semi-<diagrams based upon Mustelus mustelus as figured by Franz. 



The guanin is shown in silver. At the left: sagittal seaion through the chorioid of a light- 

 adapted eye, showing pigmented processes expanded over the proximal surfaces of the guanin 

 plates, shielding them from the light which has passed through the retina. At the right: 

 dark-adapted condition, showing pigmented processes retracted to allow the guanin to reflea 

 light back through the visual cells. 



cc- choriocapillaris; gp, gp- guanin plates; n- nucleus of guanin cell; pc- layer of migratory 

 chorioidal pigment cells; pe- pigment epithelium of retina (devoid of pigment); pp,pp- 

 pigmented processes which can be advanced and withdrawn; uc- unmodified portion of 

 chorioid (largely out of the picture above; shows ordinary flat, static pigment cells). 



distribution of tapeta as such, and of each type thereof. Only in the 

 Carnivora-Pinnipedia is it likely that a pair of large taxonomic groups 

 share a tapetal type in common by virtue of inheritance from a common 

 ancestor. 



At present, no one can arrange tapeta in any order with respect to 

 their reflection coefficients, their efficiency as mirrors. Apart from the 

 manifest superiority of occlusibility, there is only one factor in whose 



