414 ADAPTATIONS TO MEDIA AND SUBSTRATES 



its sagittal plane. The lids are smooth when the eye is open, indicating 

 that their mobility has been reduced; and complete closure is probably 

 impossible in many species. Captive porpoises have been observed never 

 to close the lids completely for more than a few seconds at a time, even 

 during sleep, Mysticete lids, on the other hand, are less modified. They 

 are moderately wrinkled when open, and can perhaps be held closed 

 without strain, though there is no more need for them to be able to close 

 than in the case of the toothed whales. The upper lid is reduced, but the 

 lower is puffy; for, the fornices have not shifted to let the eye aim down- 

 ward as readily as in the odontocetes. 



Going with its general superiority over the mysticete organ, the odonto- 

 cete eye has the pupillary operculum much better developed — this being 

 in sympathy with the predominant importance of the lower visual field, 

 as is also the dorsal location of the tapetum lucidum in all whales. The 

 operculum is actuated by intrinsic muscles, in contrast to the pupillary 

 opercula of fishes — whose modus operandi is unknown, but which at 

 least are known to contain no muscles. The internal shape and arrange- 

 ments of the cetacean eyeball are strikingly fish-like (Fig. 141), except 

 that it has not gone in so strongly for periscopy. The ovoid cornea is 

 small in area, particularly in mysticetes, which by some is considered an 

 adaptation for the conservation of heat. In the toothed whales at least, 

 the cornea is greatly thickened at its margin, as in elasmobranchs and 

 many teleosts. The eye is horizontally ellipsoidal, which helps to extend 

 the horizontal visual field. The antero-posterior axis is enabled to be short, 

 by the hardness and relative smallness of the lens, which has a refractive 

 index approaching that in fishes. In odontocetes the lens is often a per- 

 fect ball, and never has an equatorial diameter more than 1.2 times its 

 axial diameter. In whalebone whales the lens is at least this much flat- 

 tened, and may have an equatorial diameter as much as 1.5 times the 

 axial. In keeping with these differences, toothed whales have flatter, 

 hence more fish-like eyes, the axis being six-tenths of the vertical diam- 

 eter; while in mysticetes it is seven- or eight-tenths. 



The Odontoceti have powerful ciliary muscles, sometimes even with 

 some circular fibers as well as radial ones. Baleen whales may have no 

 ciliary muscle at all, and never have more than from one-half to one 

 diopter of accommodation. In compensation, they have even longer rods 

 in their retinae than the very long ones of odontocetes, though this differ- 

 ence undoubtedly exists primarily to increase sensitivity, in those whales 

 which sound to almost lightless depths. It does not appear to be known 



