AQUATIC MAMMALS 



posterior part of the manubrium. In the Sirenia, therefore, there is en- 

 countered the condition of a thorax extremely strong above and extremely 

 weak below. It accordingly seems likely that as the Sirenia do not des- 

 cend very deeply the aquatic pressure upon the thorax is relatively uni- 

 form. The thorax does not need to adapt itself for fluctuating compres- 

 sibility and the attachment of the ribs to the vertebrae is therefore less 

 elastic. A necessity in this connection would seem to be that the animal 

 breathe almost entirely by the diaphragm. 



In Sirenia the articulations of the vertebrae in the lumbo-caudal series 

 are definitely reduced, this being most marked in the manati. Anapophy- 

 ses are lacking as are also well defined postzygapophyses from this series. 

 Metapophyses are present in the manati although they are fairly fused 

 with the prezygapophyses. In the dugong there are no metapophyses 

 occurring as processes in the thoracico-lumbar series and the zygapophyses 

 do not project laterad but are flush with the bony laminae of the spinous 

 processes. In the manati the spines and zygapophyses are rugose with 

 numerous small but well defined prominences. In the dugong the spin- 

 ous processes are of moderate and approximately equal height in the 

 thoracico-lumbar series. In the manati the spines are not quite so long. 

 Murie (1872) has stated that in the manati the intervertebral substance 

 is very limited in amount, being no more in thickness than a tenth of an 

 inch, and this seems quite surprising in view of the fact that in other 

 aquatic mammals this substance is unusually generous in amount, and in 

 consideration of the extent to which this animal can curve its body and 

 tail. The latter is often used as a prop (see figure 7) to keep the body 

 from resting on the ground while feeding, and the curvature of the 

 lumbo-caudal region is then excessive. 



From Murie's excellent illustrations it is seen that the panniculus 

 carnosus of the manati is a most extraordinary muscle. Although some 

 of its fibers stretch to the fore limb, the main body of the muscle, which 

 may be as much as an inch and a half in thickness, extends from the pel- 

 vic region to below the eye, in a broad, powerful sheet (figure 26) . It 

 reaches quite to the midventral line, and passes both laterad and mediad 

 of the arm, the sphincter colli then becoming more complex over the 

 neck. Clearly this great muscle should have two functions; one as an 

 accessory belly strengthener, as pointed out by Murie, to support the 

 viscera and partly to make up for the absence of costal cartilages of the 

 usual sort. The other function is to help bend the body in swimming. 

 When one side of the panniculus is contracted there evidently follows a 

 twisting, more or less lateral motion, but when both are flexed together, 



[168] 



