56 TEXT-BOOK OF ZOOLOGY 



4. The specific weight is diminished by the thick subcutaneous layer 

 of blubber (fat being lighter than water), and also by the extreme lightness 

 of the bones of the skeleton (some of the cranial bones are almost as thin 

 as paper). 



5. The smoothness of the skin has the effect of diminishing friction in 

 the water. 



6. External ears are absent. These, if present, would impede the 

 animal's progress. Their absence does not affect the sense of hearing, 

 inasmuch as the animal spends the greater part of its life in the water, 

 which is a better conductor of sound than air, while the vibrations of 

 sound are conveyed to the internal ear through the whole body. 

 (Compare the mole.) The opening of the external ear passage is closed 

 while the animal is diving below the water. 



7. The cleft-like nasal apertures can also be closed at the will of the 

 animal. 



Hi. All the Food of the Seal is supplied by the Sea. 



1. In the pursuit of its prey it is aided by — 



(a) The rapidity and agility with which it swims, and its power and 

 endurance in diving below the surface. 



(b) Its sharp-sightedness and a fine sense of touch in the tactile hairs 

 on the upper lip. (Inquire why these two senses must be specially 

 considered in the case of an aquatic animal.) The presence of so large 

 a number of these bristles upon the divided upper lip becomes explicable 

 when we reflect that during the night, its principal time for hunting, 

 and also in the dark depths of the sea, the sense of touch is the only 

 one on which the creature can rely. 



2. As the seal, for the most part, feeds upon animals of rather large 

 size, its teeth are of the same type as those of other carnivores ; but 

 inasmuch as it swallows its prey whole, they do not exhibit the same 

 diversity of form and size as* for instance, those of the cat (which see). 

 They are, on the contrary, of almost equal size. The molars and 

 premolars are provided with very sharp points. The teeth are thus 

 excellently adapted to seizing and maintaining hold of the smooth and 

 slippery fish which chiefly form its prey. The mandible is firmly 

 articulated as in the other carnivora. 



C. The Seal as a Land Animal. 



We have seen that the seal is pre-eminently a denizen of the waters. 

 Consequently its body is adapted in a high degree to an aquatic life. 

 Still, as about a third part of its life is also passed on dry land, 

 arrangements must be provided which render such an existence possible. 



