GUANIN IN CHORIOIDAL TAPETA 243 



The guanin tapetum in the elasmobranch chorioid is occlusible, and 

 is much the most remarkable of all tapeta lucida despite the fact that its 

 owner is the most primitive living vertebrate type to have a tapetum of 

 any kind. While in other vertebrates the chorioidal pigment cells have 

 at best little ability to change their shape, the elasmobranchs have spe- 

 cialized a layer of such cells whose pigment has extraordinary migratory 

 capacity, in every way equal to that of a teleost retinal pigment cell or a 

 dermal chromatophore. The bodies of these cells form a mosaic toward 

 the inner surface of the chorioid, each with a plate-like process running 

 slantwise (over most of the area of the chorioid) to the choriocapillaris. 

 The processes thus overlap like shingles set at 45° (Fig. 97), and along- 

 side of them are flat guanin-filled cells. In dim light, the migratory pig- 

 ment retracts into the body of the cell. Light rays which pass through 

 the visual cells and the smooth, pigment-free pigment epithelial cells, 

 now strike the guanin and are thrown back through the visual cells again. 

 In the light, the migratory chorioidal pigment expands so that light rays 

 now strike pigment, instead of the guanin, and are absorbed without 

 reaching the latter. The arrangement works, it is to be noted, only be- 

 cause of the slanted position of the alternating plates of guanin and 

 retractible pigment. There is a strip of chorioid, usually horizontal and 

 always superior to the optic disc, in which the guanin plates are not 

 slanted. Here, locally, the tapetum is fixed and non-occlusible. The one 

 known area centralis in selachians — that of Mustelus (Fig. 77a, p. 185) 

 — is located within the non-occlusible region. An area centralis (for 

 acuity) and a tapetum (for sensitivity) are of course not incompatible, 

 as is obvious from the situation in the ungulates and carnivores (v. i., 

 and note, p. 185). In at least three elasmobranchs the tapetum is under- 

 standably lacking : Lcemargus is an abyssal shark, Myliobatis is a pelagic 

 ray which has cones as well as rods; and the basking shark (Selache 

 maxima) basks a good deal, as its name implies. 



Phytogeny and Relative Efficiency of Tapeta — One naturally 

 wonders which of these various types of tapeta is most effective; and, 

 if any one is outstanding, why any other types were ever produced. The 

 potentialities of all tapetal types are apparently present in the fishes; but 

 the above questions are quite impossible to answer at present. It seems 

 surprising that so ingenious a device as the elasmobranch tapetum should 

 not have persisted all the way to the mammals — or at least have been 

 re-invented one or more times. But it must be remembered that diurnality 



