THE HOLOSTEAN EYE 575 



the iris stroma. The uvea here contains no ciHary or Briicke's muscle; 

 but mid-ventrally there is a lens-muscle papilla or campanula. In Amia 

 this is large, and the pigmented lens muscle blends with a definite tendon 

 which in turn attaches to the lens capsule. In Lepisosteus the small 

 muscle attaches directly without a tendon; but there is a meridional 

 ridge (lacking in Amia) extending backward from the campanula about 

 one-fourth of the way to the posterior pole of the eyeball, along the 

 route of the old embryonic fissure. This ridge may be homologous with 

 the teleostean falciform process iv.i.), but this is not certain; nor is it 

 known for sure whether the holostean 'campanula' and lens muscle are 

 even identical in function with the teleostean structures (i. e., retractive, 

 rather than protractive like the selachian lens muscle), let alone hom- 

 ologous therewith and ancestral thereto. The lens is supported from 

 above by a squarish suspensory ligament (essentially a strap of tough 

 vitreous — cf. sturgeons), with a broad insertion (4.0mm., in Lepisosteus) 

 on the lens. 



The iris is devoid of muscles. It bears meridional folds dorsally in 

 Lepisosteus (as in rays and a few teleosts) . Its anteriormost layer is the 

 mesothelium (v.s.), following which comes a thin argentea continuing 

 that of the chorioid, and a thick, pigmented stroma. The anteriormost 

 retinal layer is heavily pigmented throughout, but the posteriormost is so 

 only in the region where it is most exposed to light. Behind the annular 

 ligament (which is semi-opaque) its pigmentation fades, so that there is 

 a gradient from the pupil to the 'ciliary' region, where the innermost 

 epithelial layer is completely bleached as in all vertebrates. 



In both genera, the vitreo-retinal boundary consists of a delicate, 

 presumably mesodermal membrane, in which is suspended a network of 

 small blood vessels. These Vitreal' or 'hyaloid'* vessels, first encoun- 

 tered here historically (but see p. 566, bottom) , are common in teleosts 

 and occur in some lungfishes and in amphibians, with imitations (of 

 entirely separate origins) in snakes and mammals. They are clearly de- 

 voted to the nourishment of the inner layers of the retina, and will be 

 discussed below in the paragraphs on the teleosts. In the holosteans, as 

 in the amphibians and some teleosts (catfishes), the large artery and 

 vein which supply the network enter the eyeball cavity at the mid-ventral 

 point of the ora terminalis. 



*No connection with the hyaloid vessels of the fetal mammalian eye — see p. 113. 



