THE SENSES 



The manner in which Putter found the eyes of pinnipeds to differ 

 from conditions in the Cetacea may be summarized as follows: The 

 pre-equitorial segment is thick and the equitorial segment thin, while 

 the latter is thick in whales; the fundus segment is thick in all. The 

 choroid is thin in pinnipeds and mysticetes, but thick in odontocetes. 

 The tapetum lucidum is poorly developed. The ciliary muscle is feeble 

 in pinnipeds, absent in mysticetes, and represented by a few fibers in 

 odontocetes. The ciliary processes are moderately long in pinnipeds but 

 short in cetaceans. The lens is larger than in whales and the rods of 

 the retina very long. Although the sheath of the optic nerve is not 

 so robust as in whales, it is much thicker than normal. The tarsus of 

 the lid, while poorly developed, is more so than in whales. Meibomian 

 glands are absent in all three. Nictitating membranes are present in 

 pinnipeds (and sirenians) but absent in whales. The eyeball in pinni- 

 peds is slightly movable, which is not the case in cetaceans. In the 

 former the axis of the eye is horizontal or directed somewhat dorsad, 

 while in the latter it is horizontal or directed somewhat ventrad. It is 

 further mentioned that of all mammals sirenians may be said to have 

 the cornea least developed. 



As Kellogg has suggested, adaptive visual changes, when the need 

 for them has arisen, are of critical importance to a marine mammal. 

 He pointed out that the atrophy of the optic chiasma in the zeuglodont 

 brain indicates a failure in their visual apparatus very possibly because 

 of an inability to adjust their vision to salt water requirements, and 

 this may very well have been the deciding factor in their eventual 

 elimination. 



Kellogg's investigations of cetacean eyes were based on mysticete 

 material and he informs me that there is not the slightest doubt but 

 that the eyes of this group are well nigh useless for seeing above the 

 surface of the water. Putter stated that the eyes of odontocetes and 

 mysticetes differ in many important respects and to a degree that indi- 

 cates that in each the eye has experienced its own particular type of 

 specialization from a more generalized optic equipment. Having never 

 worked on this question myself I am in no position to point out any 

 erroneous conclusions which this author may have reached, but it 

 seems that he was mistaken in ascribing to mysticetes, odontocetes and 

 pinnipeds a common inability to see effectively through an atmospheric 

 medium, very likely for the reason that apparently he did not determine 

 mathematically the refractive powers of their visual equipment. At 

 any rate it is well known that when a watchful seal is basking it is 

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