AQUATIC MAMMALS 



more diverse in their affinities than are the cervical elements of the spinal 

 system and they could never become as simplified as the latter. 



To recapitulate briefly; it is believed that the long, flexible neck of 

 the otariid is essential to its special method of swimming. The dif- 

 ferent method of swimming employed by the phocid and cetaceans ne- 

 cessitates a different cervical conformation. Although feeding habits of 

 the seal may require a neck of considerable length, its effective length 

 while swimming is reduced by the small size of the head and great 

 breadth of the neck base, as well as by as much contraction as possible. 

 The larger size of the head in whales requires marked reduction in neck 

 length, in the porpoise so as to bring the head nearer the thorax, and pos- 

 sibly in balaenid and sperm whales so as to bring the thorax nearer the 

 head and mechanically to make of the cervical series a single element 

 comparable to a thoracic vertebra without ribs. 



[152] 



