LOCOMOTION AND LOCOMOTORY ORGANS 



t05 



appreciated by comparing the skeleton of a whale with that of a terrestrial 

 mammal (Fig. 21). Cetaceans have a full complement of seven cervical 

 vertebrae as found in most mammals irrespective of whether their neck 

 is as long as the giraffe's or as short as the dolphin's, but while Rorquals, 

 River-dolphins, Belugas and Narwhals still have independent cervical 

 vertebrae, Right Whales, Dolphins and Porpoises have all the seven 

 vertebrae fused into one osseous unit (Fig. 64). The resulting short and 

 rigid neck not only adds to the streamlining effect of the I'est of the body, 

 but also aids propulsion in other ways for, since the motive force lies in the 

 tail, a less rigid head would flop in all directions and impede the animal's 

 progress through the water. 



The rest of the vertebral column also shows characteristic modifications 

 to aquatic life. Even a superficial examination of Fig. 21 will reveal the 

 extraordinary extension of the lumbar part of the vertebral column. This 

 may be partly associated with the formation of a streamlined body, but 

 it is also connected with the fact that it is to the lumbar vertebrae that 

 the tail muscles are attached. A long tail which is expected to develop 

 a great deal of power must naturally lead to an increase of the surface 

 to which its muscles are attached. In some species, e.g. the snake-shaped 

 Basilosaurus (Fig. 28) and also - though to a lesser extent - in ^iphiidae (e.g. 

 the Bottlenose Whale), increase in body length goes hand in hand with 

 longer vetebrae. At the same time these animals have a shortened 

 thorax. In most other Cetaceans, however, the lumbar region has become 

 extended through increases in the number of individual lumbar vertebrae. 



Figure 63. Forces operating during 

 vertical motions of a Cetacean tail. 

 The force F resulting from the resis- 

 tance of the water can be resolved 

 into a forward (Ff), and into an 

 alternatively up {Fu) and downward 

 {Fd) component. 



Upstroke 



