138 



WHALES 



Figure 80. Cross-section of the thorax of three terrestrial mammals [man, horse and dog) 



contrasted with that of three Cetacea {Porpoise, Greenland Whale and Archaeocete) . The 



figure shows, inter alia, that the shape of the thorax of Archaeocetes was closest to that of 



quadrupedal terrestrial mammals. (Slijper, lyjS.) 



I should like to draw attention to the fact that the differences between 

 Cetaceans and terrestrial mammals rest mainly on three factors: the 

 peculiar barrel-shape of the Cetacean thorax, the dorsal position of the 

 lungs, and the peculiar angle of the diaphragm. The barrel-shaped 

 thorax (which in terrestrial animals is much narrower in front) is con- 

 nected with the general stream-lined form of the body in which neck and 

 body have become fused, and with the fact that the fore-limbs, not having 

 to support the body, can be placed more laterally. The dorsal position of 

 the lungs, and also the fact that they thin out towards the ventral side, 

 is, as we have just seen, associated with stability. As a result, the dorsal 

 part of the thorax is wider than the ventral part. (The reverse is true of 

 terrestrial mammals.) In cross-section, the thorax of Odontocetes therefore 

 looks heart-shaped, while that of Mysticetes and Archeocetes is fairly 

 circular. The peculiar slope of the diaphragm (Fig. 79) is a direct 



