126 TEXT-BOOK OF ZOOLOGY 



halves of the mandible are only united by ligaments, and only the first 

 pair of ribs is directly united with the sternum ; all these arrangements 

 tend to prevent fracture of the bones, and impart great elasticity to the 

 body. 



3. All land mammals possess as a protection against injury a more 

 or less thick upper skin or integument. (Compare elephant and rhi- 

 noceros.) The skin of the whale, on the other hand, is remarkably thin, 

 and as soft as well-greased leather (see Section 5, i.). (Compare in this 

 respect the softness of the body of many aquatic animals.) 



4. The whale has no hairy covering like that which protects land or 

 amphibious mammals against excessive loss of body-heat (see p. 11) ; 

 such a covering would be indeed utterly useless to the animal. (Why ? 

 See seal.) (A few bristles may be found here and there in exceptional 

 cases.) On the other hand, the body is completely enveloped in a 

 subcutaneous layer of fat (blubber) from 8 to 16 inches thick. (Compare 

 with seal.) This excellent means for the preservation of heat enables the 

 animal to inhabit even the ice-cold waters of the Arctic Seas. 



5. The movements of the whale are remarkably nimble and rapid. 

 In a few seconds it is able to escape from the reach of its pursuers. 

 The animal may often be seen tumbling about in the water in play, 

 suddenly lifting its gigantic body out of the waves, or standing on 

 its head and lashing the water with its tail. This speed and extra- 

 ordinary nimbleness is due to the following structural arrangements of 

 the body: 



(a) The body has the form of a boat, like that of a fish. (Compare 

 with seal.) For this reason the whale used to be erroneously classed 

 with the fishes, and in German still goes by the name walfish, i.e., 

 whale-fish. 



(b) The head is united to the trunk without any visible neck. (Com- 

 pare seal.) The cervical vertebrae are indeed present as in other mammalia, 

 but they form only narrow rings or discs, which, moreover, are partially 

 united with each other. 



(c) Whilst in the seal the hind-limbs act pre-eminently as organs of 

 locomotion, in the whale they (as well as the pelvis) are reduced to a 

 few minute bones embedded in the flesh. The organ of locomotion of 

 the whale is 



(d) The tail, which is actuated by powerful muscles, and forms a 

 gigantic fin. By its rotatory movements it acts just like the screw of a 

 steamer in driving the animal forwards, while by its upward or downward 

 strokes it enables it to sink below or ascend above the surface. The 

 force located in the tail may be estimated from the fact that one stroke 

 of it can shatter a stout boat. 



