160 ADAPTATIONS TO A RHYTHMIC ACTIVITY 



The sturgeons are elasmobranch-like in habits as well as otherwise. 

 Their pupils (Acipenser fidrescens, Scaphirhynchus platorynchus) are 

 broad, pointed vertical ellipses or rhomboids, which appear to move only 

 passively as the lens blocks or unblocks them (in accommodation ?) . The 

 other chondrosteans, and the holostean fishes, have not been studied. 

 The lungfishes have no iris muscles although one (Protopterus) has a 

 mobile pupil. Here the contractility of the unmodified epithelial cells of 

 the pars iridiaca retinae seems to be involved, as possibly also in a few 

 teleosts. 



Among the teleosts only the eels and the flattened upward-lookers 

 (e.g., Uranoscopus and Lophius) have much pupil excursion. The com- 

 mensal pearl-fish, Encheliophis (=Fierasfer) jordani, also has the eyes 

 aimed upward and is highly exceptional in that its pupils can close to a 

 mere dot. Many of the flounders and their relatives, and one or two arm- 

 ored catfishes, have a pupillary operculum (Fig. 65). Most teleosts have 

 no functional iris muscles whatever, though the non-contractile 'sphincter 

 of Grynfeltt' is often present. In atypical eyes, like those of Periophthal- 

 mus, a functional sphincter may be present without a dilatator. The lens 

 bulges far through the pupil except when pulled backward in accommo- 

 dation, but does not necessarily actually block it, for in many species 

 a narrow 'aphakic' (lenseless) space surrounds the lens. The teleost iris 

 is usually so anchored to the cornea by the so-called annular ligament 

 (more properly, 'iris angle tissue' — Fig. 67, al) that any iris muscles 

 would be powerless to alter the pupil. In the minority which do have 

 iris muscles (and weak annular ligaments) these are peculiar in that the 

 dilatator consists of true, discrete muscle cells lifted free of the posterior 

 epithelium and embedded, like the sphincter, in the stroma. 



Most teleosts thus depend upon retinal photomechanical changes, which 

 were evolved by these fishes or by their holostean ancestors. The retinal 

 movements control stimulation well enough and the animal does not miss 

 the stopping-down effect of a contractile pupil because of the peculiar 

 optics of its eye. One would expect the spherical lens to have an excessive 

 degree of spherical aberration and to need more stopping than the flatter 

 lenses of terrestrial forms. But the teleost lens has a radially graduated 

 index of refraction and the retina is a spherical surface, concentric with 

 the lens. Hence the retina receives a sharply focused image from any 

 angle and the eye is in effect both periscopic and aplanatic. Constriction 

 of the pupil would serve no useful purpose. Indeed, the pupil margin 

 need not overlap the lens at all; and the iris may even be lacking, as in 



