THE MONOTREME EYE 



667 



are sent into the cornea for some distance, marks the boundary. No- 

 where else above the teleosts is it so readily to be seen that an outer 

 portion of the substantia propria is homologous with the dermis rather 

 than with the dura (Fig. 194a; cf. Fig, 151, p. 451). A Bowman's mem- 

 brane has been claimed for Ornithorbynchus, but none can be made out 

 in Tachyglossus. Both these genera have the usual Descemet's layers, 

 but Kolmer could not make out the elastic membrane in Zaglossus. 



Fig. 194 — The monotreme eye. 



a, sertion of eye of Tachyglossus sp. x8. Drawn from a preparation of O'Day. 



b, inner surface of segment of anterior uvea of Tachyglossus. After Franz. 



av- anterior surface of vitreous; cd- conjunctival dermis; ch- chorioid; cp- ciliary process; 

 cs- canal of Schlemm; cw- ciliary web; ev- episcleral vessels, marking boundary between 

 dermis and fibrous tunic; »'- iris; /, /- external limits of cornea; m, m- rectus muscles; 

 ot,ot- ora tertninalis retina; r- retina; s- sphincter; sc- scleral cartilage (black); s\- fibrous 

 layer of sclera; ^- zonule (main portion). 



In keeping with its aquatic habits the duck-bill has a relatively broader 

 cornea than Tachyglossus, but it has a deeper anterior chamber (cornea 

 4.0mm. in diameter in a 6.0mm. eye, vs. 3.4mm. in a 8.0mm. eye; cham- 

 ber 1.25mm. deep ys. 0.9mm.). The duck-bill's corneal substantia propria 

 is only one-fourth as thick as the echidna's, but its epithelium is much 

 thicker and nearly equals the propria — such thickening being highly 

 characteristic of aquatic vertebrates in general. The duck-bill cornea is 

 lOOfl thick peripherally, only 55 [i apically. Zaglossus reverses this rela- 



