AQUATIC MAMMALS 



cestors or not. Furthermore, we lack a proper yardstick in that there is 

 no means of teUing in a particular case whether an apparent slight 

 shortening of the neck is real or due entirely to a relative elongation of 

 the trunk. Of one thing, however, we can be reasonably sure in most 

 instances. In perhaps a majority of large terrestrial mammals a primary 

 regulator of neck length is limb length, for the neck must usually be long 

 enough to reach the ground. Aquatic mammals lack this stimulus and 

 we can feel reasonably sure that this provides one factor for neck shorten- 

 ing. It seems that the only factor for decided elongation of the neck 

 that could possibly occur for this group would be in the case of one with 

 a slow-moving body which found it of preponderant advantage to secure 

 very active prey by darting movements of a mobile neck. Unless its prey 

 be of marked agility then the aquatic mammal finds it as easy to turn 

 the head by a shift of the entire body as to bend the neck. This is 

 certainly so in the case of Sirenia, and it is a stimulus for cervical shorten- 

 ing, probably to a greater degree than in most aquatic mammals, for these 

 beasts are so sluggish that feeding habits are likely a stronger stimulus 

 for neck length than those arising from locomotion. 



The above remarks suggest that the sirenian neck should be definitely 

 but not excessively shortened, and this it is. Additional evidence is 

 furnished by the fact that occasionally in the manati two or three of the 

 cervical vertebrae are fused, the second, third, and fourth then being the 

 ones involved. This is one of the only two living mammals having but 

 six vertebrae in this series. Murie (1872) considered that it is the third 

 cervical that is missing because of the conformation of a slip of the 

 scalenus muscle, while several others have considered that it is the sev- 

 enth which is lacking. Considering the phenomenal regularity with 

 which just seven cervical somites are laid down in the case of mammals 

 I would be extremely loath to believe, without extremely strong evidence 

 furnished by the conformation of the cervical nerves, that Trichechus 

 constitutes an exception to the rule. Rather do I prefer for the present 

 to assume that this genus has but six cervicals for the reason that the 

 thorax long since shifted forward and took unto itself the seventh cervi- 

 cal. In fact it seems that the dugong may even now be undergoing this 

 process, for occasionally if not invariably there is a pair of rudimentary 

 ribs attached to the seventh cervical. This brings to mind the possibility 

 that the stimuli for a short neck in the Sirenia may have been stronger 

 than all facts now indicate, but that this order has inherently lacked 

 the ability to respond to them as readily as have the Cetacea. According 

 to the literature there is some question whether Hydrodanialis had six or 

 seven cervicals. 



[140] 



