380 ADAPTATIONS TO MEDIA AND SUBSTRATES 



of the eyeball, and to a disturbance of its optical performance, if they 

 were not somehow minimized. 



The reduction of friction and of the asymmetry of pressure is partly 

 effected by the ellipsoidality of the eyeball. The visual axis of the fish 

 eyeball, as we have seen, is almost always its shortest diameter. Its hori- 

 zontal, cephalo-caudal diameter is commonly its greatest dimension, and 

 may exceed the vertical diameter by fifty per cent or more (Fig. 104, 

 p. 259) . Thus the pelagic fish eye, partly for the sake of streamlining and 

 partly for the sake of a wide horizontal visual angle — which swift fishes 

 of course desire — is not a ball, but is rather an 'ellipsoid of revolution'. It 

 presents to the water a portion which, as to curvature, is shaped like the 



Fig. 130— Scleral ossicles in fishes. After Edinger. 



a, an arthrodire, Dinichthys gouldii, exemplifying the four-part ring characteristic of many 

 ancient fishes, x J4 b, pike, Esox lucius; scleral cartilage and the two ossicles character- 

 istic of modern teleosts. x 1. c, tuna, Thunnus thynnus, showing return to complete ring 

 (which, however, involves but two ossicles), x Vi. 



bowl of a teaspoon; and, of course, the part of the eyeball which shows 

 through the lid-opening can still be, and often is, quite circular in out- 

 line — just as we can easily cut a circular piece out of a teaspoon. Past 

 the teaspoon-surface, which is often a part of the head-surface itself, the 

 water may stream with the least possible distortive action. The cornea 

 may be rendered violently astigmatic by its dual curvature (see Fig. 13, 

 p. 28) , but since its surface is under water, no optical harm is done. The 

 spherical lens, alone, is forming the image on the retina. 



In lampreys the ellipsoidality or horizontal elongation of the eyeball 

 is very slight, but the eye is smoothly covered by the primary spectacle 

 anyway (see section D). Ellipsoidality is very marked in many elasmo- 

 branchs, which are often swift swimmers as their predatory habits natur- 



