438 



THE EYE IN EVOLUTION 



Spotted cuscus, 

 Phalanger 

 mactdatus 



The eyes of Marsupials represent a transition between the wholly 

 rejDtilian-like eyes of Monotremes and the mammalian-like eyes of 

 Placentals. The globe is spherical and the sclera fibrous like that of 

 snakes, the ciliary musculature shows a reptilian ancestry but the 

 structures accessory to reptilian accommodation have all been lost, 

 the retina may have a vascularization either of the reptilian or mammal- 

 ian type, a retinal tapetum as occurs in some Reptiles may be present, 

 and the visual elements, closely resembling those of Monotremes, are 

 tyjjically rej)tilian. 



Fig. 549. — The Eye of the Wallaby ( x 5) (O'Day). 



Dasyure, 

 Dasyurus 



Tasmanian devil, 

 Sarcophilus 



THE GLOBE OF THE EYE is Spherical or almost sj^herical, with a 

 large cornea and a fibrous sclera without cartilaginous or osseous 

 suj)ports ; the marsupial mole, Notoryctes, has cartilaginous nodules 

 in the sclera. There is no Bowman's membrane but a thick Descemet's 

 membrane. The choroid is of the mammalian type with, in a few 

 species, a tapetum fibrosum (the flying phalanger, Petaurus, and some 

 of the Dasyuridse — the cat-like Dasyurus, the Tasmanian wolf, 

 Thylacinus, the Tasmanian devil, Sarcojjhilus). In Dasyurus this 

 extends over the entire funchis but is fimctional only in the upper half 

 where the retinal epithelium is devoid of pigment. The ciliary body 

 is well formed and provided with j^rocesses, and a ciliary musculature 

 is always present despite the fact that no accommodation has yet been 

 "nnstrated in any member of the group (Figs. 550-1). Sometimes 

 3 disposed as in Reptiles, com2:)rised of a meridional muscle (of 



