410 ADAPTATIONS TO MEDIA AND SUBSTRATES 



animals are described as being nocturnal, and their eyes are built for 

 sensitivity (though there is no tapetum lucidum). The pupil is large, 

 and is known to react promptly and to have a considerable excursion. 

 It is said to be displaced ventrally; but its shape is in dispute: it has 

 been called horizontally oval in both Dugong and Trichechus, circular 

 in both fetal and adult dugongs, round in living manatees and horizon- 

 tally oval in dead ones. Except for a few capillaries around the small, 

 round disc, there are no retinal vessels (as in Rhinoceros) , suggesting a 

 low retinal metabolism and implying a pure-rod condition or the pres- 

 ence of but few cones at best. The visual cells have never been preserved 

 well enough to be described accurately, but the ganglion cells are so few 

 that summation must be great; and though the optic nerve is thick, much 

 of its thickness is sheath. 



The high sensitivity which doubtless exists is presumably required by 

 the murkiness of the water stirred up in feeding, but it has been gained 

 at such an expense of visual acuity that accommodation has been dis- 

 carded as valueless and even static optical relationships have been 

 allowed to come undone. All observers agree that the vision of the 

 sirenians is wretched, and that they pay no attention to visual stimuli 

 except to withdraw from a bright light. The low value of their eyesight 

 to them is underlined by the fact that although they almost never look 

 through air, they actually have less refractive error in that medium than 

 in water (v.s.). All in all, though the rest of the sirenian body is pro- 

 foundly modified for full-time marine existence, the eyeball is a dis- 

 appointment. If we had only the sea-cows to go by, we should be forced 

 to conclude that the mammalian eye is too set in its ways to depart far 

 enough from them to give a passable imitation of the eye of a fish. 



Whales — The whales have done much better. These great mammals fall 

 into two sub-orders, the Mysticeti or baleen whales and the Odontoceti 

 or toothed whales. The Mysticeti have specialized their feeding mech- 

 anism for straining masses of plankton organisms (largely 'krill', shrimp- 

 like crustaceans) out of great volumes of water forced through their 

 plates of baleen ('whalebone') by the inflatable, connective-tissue tongue. 

 In other respects they are not more highly specialized, except as to size 

 itself, than the Odontoceti. Although the mysticetes had toothed ances- 

 tors which were already whales, these were not odontocetes. The extinct 

 zeuglodont whales do not appear to have been the ancestors of either of 

 the existing groups of Cetacea, and these ancestors have yet to be found. 



