RHINEURA. 



51 



(a) The Iris. — In the structure of the irideal region the eye of this species is 

 unique among the degenerate vertebrate eyes so far described. In all other eyes, 

 with the possible exception of Troglichlhys, elements of an iris are distinctly recog- 

 nizable. In Rhineura the fold of double epithelium between the pigmented and un- 

 pigmented part of the retina whose margin is the margin of the pupil has been 

 obliterated and the pupillary edge forms the extreme outer edge of the blunt end 

 of the' pear (p, fig. 19 a). The pigmented layer of the retina in other words merges 

 directly into the unpigmented layers of the retina. The entire thickness of the 

 retina is thus exposed at the distal face of the eye. 



Fig. ip. (a) Horizontal Section of Left Eye of Specimen, 280 mm. long. 



lb) Another Section through Same Eye, showing Exit of Optic Nerve, the Pigmentless 



Condition over Anterior Face of Eye, and Invaginated Pigment at End of Pear, 

 (c) Outline of Pigment in Proximal End of Right Eye of same Individual, showing 

 Invagination of Pigment to form a Cyst. 



