MAMMALS 431 



The eyes of these three tyjjes differ considerably, those of the 

 first two, particularly the Monotremes, exhibiting many features 

 characteristic of their reptilian ancestors adapted for nocturnality. 



THE MONOTREME EYE 



THE MONOTREMES are the most primitive of Mammals and include two types 

 (Figs. 536-7) : the duck-mole or duck-billed platypus {Ormthorh yncltus) , found in 

 the rivers and lakes of Australia and Tasmania, a shy creature with an enormous 

 fiat bill, which spends most of its time grubbing for small animals in the muddy 

 bottoms ; and the spiny ant-eaters (the echidna, Tachyglossus, found in Australia, 

 New Zealand and New Guinea, and its near relative, Zaglossus, found only in 

 New Guinea), nocturnal ant -eating creatures burrowing in rocky regions. 

 Neither relies primarily on vision ; the platypus relies largely on hearing, the 

 eyes being closed when submerged, but the vision appears to be acute during 



Fig. 538. — Diagram of a Monotreme Eye. 



A, small annular pad ; Ch, choroid ; ON, optic nerve ; S, scleral cartilage; 

 Sc, sclera ; SM, sjDhincter muscle ; VS, ciliary venous sinus. 



the twilight hours. Vision can be only of secondary importance to the nocturnal 

 ant-eater with its keratinized cornea. 



The monotreme eye has many affinities with the eyes of Rejjtiles 

 which it resembles much more than the tyj)ical eye of Mammals ; the 

 eye, indeed, is that of a reptile in a mammal. There are only two 

 outstanding differences between it and the reptilian eye. The first 

 concerns the intra- and extra-ocular musculature, the former being 

 confined to a spliincter of smooth fibres, the latter including a superior 

 oblique muscle arising from the apex of the orbit. In the second place, 

 the (otherwise typically rei)tilian) retina is completely avascular 

 without any cone-like structure nor any participation of a hyaloid 

 system in its nutriment. There has, however, been comparatively little 

 work devoted to the subject — Marcus Gunn (1884) (the specimen sent 

 from Australia to London preserved in Scotch whisky), O'Day (1938- 

 52) and Newell (1953) on Ornifhorhynchus, and Owen (1868) (macro- 



