AQUATIC MAMMALS 



whales. From examinations of many freshly killed specimens of the 

 latter I am convinced that it is so, for the lids may be moved with the 

 fingers very easily; but Putter has stated that they are immovable, as 

 is also the case in odontocetes. It is true that in at least the majority 

 of toothed whales the lids are unwrinkled when the eye is open. 

 It therefore seems likely that their mobility is very much reduced. 

 Complete closure of the eye is perhaps impossible, but I would be 

 loath to believe that the lids are without power of movement entirely. 

 The narwhal (Monodon), and perhaps some others, has a very peculiar 

 modification of the eyelids, which will be described in detail by Ernst 

 Huber. 



An optic tendency resulting from a life in the water is for the elim- 

 ination of eyelashes, but their disappearance is slow. Lashes are en- 

 tirely lacking in the Cetacea and I understand that they are practically 

 so in the Sirenia. All other aquatic mammals have them in various 

 degrees. One might expect that there would be a great advantage in 

 the acquirement of a nictitating membrane, but the only sorts which 

 have them are the Sirenia and Pinnipedia, so far as I have been able 

 to learn. There should hardly be any use for a functional lachrymal 

 duct in aquatic mammals and this shows a tendency to disappear (Ceta- 

 cea, Sirenia and Pinnipedia) . 



Frequent or constant submergence will probably bring about some 

 sort of alteration in composition of the lachrymal fluid in order that 

 this will not so readily be washed from the eye, and very possibly in 

 order to counteract any irritative influence of the sea water ; and Putter 

 has determined that this is actually the case. It should be noted in 

 this connection that cetaceans are perhaps the only mammals which 

 cannot rub the eye against some part of the body in order to free it of 

 foreign particles, including parasites. 



Very probably there need be no optic stimulus in addition to the 

 above for aquatic mammals inhabiting fresh water. The refractive 

 index of the latter is not so different from that of air but that the 

 normal eye can see well beneath the surface and there is no need for 

 it to withstand any considerable pressure. In a marine mammal such 

 as the whale, however, matters are entirely different. The refractive 

 index of sea water differs so much from that of fresh or of air that 

 when the normal eye is submerged in it very little can be done but to 

 distinguish objects by their high lights and shadows, because of the 

 fact that the point of focus for the image then lies far behind the 

 retina. Hence, for a whale to see properly when submerged means 



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