THE SENSES 



It is thus seen that mammals which inhabit fresh water usually re- 

 tain the pinna, while pelagic mammals have mostly lost it, both be- 

 cause they are more highly aquatic and because they have less need for 

 it. It is retained by the sea otter and Otariidae, but in the latter 

 the pinna is but a remnant and too slender to act as an acoustic aid. 



If an aquatic mammal show by the position of its eyes that it has 

 experienced a need for peering above the surface of the water with the 

 minimum of its body exposed to view — in other words, if its eyes have 

 assumed a somewhat dorsal direction, then has it similarly experienced 

 a need for hearing in this position, and the external orifices of its ears 

 will also be situated more dorsal than usual. This is not apparent in 

 any insectivore and is at all marked among rodents only in those aqua- 

 tic forms having a rather flat skull, in which the external auditory meatus 

 is relatively close to the dorsal profile (as Castor), and in such this de- 

 velopment may have nothing to do with an aquatic modification, as 

 many terrestrial rodents exhibit the same character. The well formed 

 ears of the hippopotamus are markedly dorsal in position. In all Pin- 

 nipedia the auditory lumen does not extend directly laterad from the 

 meatus but turns quite sharply upward, reaching the body surface con- 

 siderably dorsad of the meatus. This is most marked in the Phocidae 

 and least so in the Otariidae, and is an expected condition. I strongly 

 suspect that this is also the case in the Sirenia. In the Mysticeti (at 

 least in Balaenoptera) the auditory tube extends practically in a direct 

 lateral line to the surface of the body. At least in Neomeris, among the 

 Odontoceti, there is a sharp bend of the lumen upward for about an 

 inch. If this should prove to be a uniform character among the toothed 

 whales it should be of considerable phylogenetic importance. 



As already mentioned the ability to close the ear so as to exclude 

 water is one of the first acquirements of an aquatic mammal and it is 

 confidently believed that all mammals that may be so classed have it. 

 It is one which is easy of accomplishment. Most terrestrial mammals 

 which burrow also have it, for the purpose of keeping loose soil out of 

 the ear. Perhaps most often this closure is accomplished by the pulling 

 into the orifice of a valvular plug, probably homologous with the anti- 

 tragus, specialized for this purpose. This may be seen to excellent ad- 

 vantage in the case of the seal (Phoca) but the precise functioning of 

 the mechanism is not uniform and the result may be attained in different 

 genera, and even in well differentiated species, by complex variations 

 in the actions of the small auricular muscles, which are relatively very 

 well developed. A somewhat difl^erent method for closure is employed 



[69] 



