THE CHONDROSTEAN EYE 571 



the unmodified chorioidal layers somehow getting in between them. In 

 all probability, however, the sturgeons' argentea and tapetum were quite 

 separate inventions despite their superficial histological resemblance. The 

 argentea clings to the sclera even over the bump formed on the chorioid 

 by the suprachorioidal cushion, and extends into the iris. Here it splits 

 up into many lamellse which occupy the whole thickness of the iris stroma 

 and are sandwiched between layers of stromal connective tissue. The 

 vascular supply of the eyeball, which relates solely to the uvea, has not 

 been well worked out; but it apparently resembles somewhat the arrange- 

 ment in elasmobranchs. 



A 'ciliary body' can be recognized, with a little effort, between the 

 ora terminalis and the portion of the uvea which is unmistakably freed 

 from the fibrous tunic to form the iris ; but its uveal portion is amuscular 

 and differs in no important histological respects from the chorioid proper. 

 In some European forms, this narrow zone is said to have meridional 

 folds; but in A. julvescens it is smooth. In all species, however, there is 

 a mid-ventral papilla whose structure and homologies remain to be fully 

 elucidated. Though it has been compared both with the elasmobranch 

 lens-muscle papilla and with the teleostean campanula, it is not actually 

 known to develop, embryologically, after the fashion of either. It appar- 

 ently contains no muscle fibers, for Hess was unable to elicit any accom- 

 modatory changes in sturgeon eyes under electrical stimulation. The 

 slightly flattened lens (Fig. 167) is suspended by a ligament quite like 

 that in teleosts (see Fig. 105g, p. 261; Fig. 169, p. '^77). 



The iris is devoid of muscles, so that if we imagine the sturgeon eye 

 to have evolved rather directly from one like that of a modern shark, 

 we must say that it has reverted to the muscle-free condition of the lam- 

 preys — owing to the adoption of the bottom habit, with a renunciation 

 of any shark-like tendency to bask (which would call for a mobile pupil 

 to protect the sensitive retina developed for the benthic mode of life) ; 

 and with a discard of accommodation, this being of no value to a 

 scotopic eye with its crude resolution. In different quadrants* the pig- 

 mentation and the argenteal content of the iris stroma vary reciprocally, 

 as if either reflection by the argentea or absorption in pigment were 

 alone adequate to prevent light from getting through the tissue. But in 

 regions where the iridic argentea is conspicuous, the distribution of 



* Unfortunately the writer cannot be more specific, for the plane of section of his material 

 {A. fulvescens, prepared by the late Harold D. Judd) is uncertain. 



