436 



THE EYE IN EVOLUTION 



the two epithelial layers and some radial blood vessels lying in loose 

 connective tissue. The sphincter muscle, comprised of the unstriated 

 fibres characteristic of Mammals, is massive ; it constitutes the only 

 intra-ocular muscle for a dilatator or cihary musculature is absent, nor 

 is any accommodative mechanism present. ^ 



Except in the aquatic platypus, the lens is relatively small and 

 flat and the zonular fibres, arising from the coronal zone of the ciliary 

 body, are inserted into its equator. In this region the subcapsular 

 epithelium is tall, twice as tall as at the anterior pole, to form a 

 miniature annular pad, a characteristic of Reptiles (Fig. 543). 



The retina is entirely avascular, dependent on the choroid for 

 nutriment. Ophthalmoscopically the fundus of the echidna is of a 



Fig. 547. — The Visual Elements 

 OF THE Platypus. 



Showing double and single cones 

 (the latter in the centre) and the long 

 slender rods (O'Day). 



Fig. 548. — A Visual Element from 

 THE Pure-rod Retina of the 

 Echidna (O'Day). 



uniform brownish colour with a chalky -white oval optic disc from which 

 nerve fibres radiate ; it thus closely resembles a common sa-uropsidan 

 type (Johnson, 1901) (Plate XIII, Fig. 1). The visual elements are 

 sauropsidan in character : the platypus has a duplex retina, the rods 

 and cones being in approximately equal numbers. The cones are both 

 single and double with oil-droplets in the former and in the chief 

 member of the latter, but with no paraboloids ; the echidna has a 

 pure-rod retina with no oil-droplets (Figs. 545 to 548). In neither 

 genus is there evidence of an area centralis or a fovea. In the optic 

 nerves of the platypus there are some 32,000 fibres (Bruesch and Arey, 

 1942). 



THE OCULAR ADNEXA are sauropsidan apart from the extra-ocular 

 muscles. The lids are thick and well-formed ; the echidna has a 



' A dilatator is also absent in Crocodilians and Marsupials. 



