xxvi MOVEMENT 667 



tion of the skin by deformation absorbs part of the turbulence energy 

 and reduces drag. The surface is completely smoothed off by the loss 

 of all hair, except for a few sensory bristles round the snout in some 

 species. There is a thick layer of dermal fat (blubber), which, besides 

 acting as a heat insulator, may also provide a reservoir of food and 

 perhaps, when metabolized, of water. The fat also reduces the specific 

 gravity of the animal and perhaps provides an elastic covering to allow 

 for changes in volume during deep diving. There are no glands in the 

 skin. 



The propulsive thrust comes largely from the horizontally placed 

 tail flukes, which constitute a cambered aerofoil that is moved up and 

 down by the tail muscles. These consist of upper and lower sets of 

 longitudinal fibres inserted in the tail-stock vertebrae by means of 

 long tendons from muscles originating from more forwardly situated 

 vertebrae. More caudally placed muscles inserted on the hindmost 

 vertebrae in the tail flukes allow movement of the flukes relative to the 

 tail stock to produce the forward thrust. The arrangement allows the 

 whole tail to be bent up and down on the body, while the fluke is bent 

 relative to the tail and produces the thrust. Stability is provided by the 

 paddle-like fore-limbs, and there is often a large dorsal fin, especially 

 in fast swimmers such as the killer- whales (Oirinus). The plasticity of 

 animal form is shown by the fact that the flipper is a modification of 

 the ambulatory fore-limb, whereas the dorsal fins and tail flukes are 

 'neomorphs', folds of skin with no skeletal support, radical innovations 

 indeed: it is not easy to imagine by what alterations of habit an early 

 eutherian could come to develop fins out of such folds of skin. Equally 

 remarkable has been the disappearance of the hind-limb, leaving no 

 external trace and internally only paired pelvic vestiges with addi- 

 tional bony nodules in some whales representing limb bones. These 

 rods serve as attachments for the corpora cavernosa of the penis and 

 may therefore be regarded as ischia. 



The vertebral column (Fig. 437) carries no weight except when the 

 animal jumps out of the water. In the vertebral column the zyga- 

 pophyses are reduced, and the centra are well developed to make a 

 compression strut, as in fishes. The vertebral epiphyses remain separate 

 to a late age. The neural spines and transverse processes are well 

 developed for the attachment of muscles in a typically mammalian 

 manner giving a dorso-ventral movement of the body, this being in 

 contrast with the lateral movement produced by the segmental myo- 

 tome arrangement of fishes. The neck is very short and the cervical 

 vertebrae partly or wholly fused together. The ribs, as in other aquatic 



