AQUATIC MAMMALS 



by the hippopotamus, for it may be seen that directly preceding sub- 

 mergence this animal contracts the entire base of the ear, which evi- 

 dently compresses and shortens the lumen. The extremely vigorous 

 and repeated twitching of the ears upon its reappearance is perhaps in- 

 dicative that the closure is not directly at the orifice, but slightly deeper, 

 so that some effort is necessary to dispose of lodged water. 



Still another way of closure is employed by the sea-lions. The car- 

 tilage is very slender and somewhat furled, and muscular action results 

 in further and tighter furling, and perhaps some longitudinal contrac- 

 tion of the lumen. For this method of excluding the water at least 

 some remnant of the external pinna would seem to be of distinct ad- 

 vantage, and it is not unlikely that this is the sole reason for the reten- 

 tion of a part of the external ear by otariids, while the lack of such 

 need has hastened its disappearance in phocids and the walrus. 



Among Sirenia I have had an opportunity for examining only a 

 hardened dugong. The auditory aperture in this was relatively smaller 

 than in a seal, but larger than in the Cetacea, and appeared to be fairly 

 intermediate in other respects. 



At first, in the early history of a hypothetical aquatic phylum, the 

 closed position of the ear would be retained with some muscular exer- 

 tion, and later with ease. A point would inevitably be reached, at a 

 time when the mammal had ceased to leave the water for more than 

 short periods, spending the greater portion of its time with the ears be- 

 neath the surface, when the action of the auricular musculature had so 

 changed that the closed position would be the one maintained involun- 

 tarily, while muscular effort would be required to open the ear. Whether 

 or not any pinniped has yet attained this stage is unknown. 



Thence it would be but a relatively short time until, through disuse 

 of the musculature involved, the ear could not be opened at all, the au- 

 ditory lumen would first remain permanently water-tight, and because 

 its apparatus for closure had throughout a long period been operating 

 with but little or no opposition from the opening musculature, its cali- 

 ber would be reduced. This in effect, is just what is now to be found in 

 the Cetacea. I have introduced a match stick into the auditory tube of a 

 Balaenoptera 75 feet long, but it would not accommodate anything 

 larger; and in some odontocetes it is even smaller. In the latter, as 

 would be expected because of the continued action here in the past of 

 the musculature for closure, the lumen is smallest at the distal end, ex- 

 panding to a slightly greater diameter proximad. In the Mysticeti the 

 external orifice is also small, the lumen thence expanding for a short 



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