GEOGRAPHICAL DISTRIBUTION OF ANIMALS. 535 



certain epochs of the year there are developed in the sub- 

 stance of these shapeless masses, ovoid or spherical corpuscles, 

 which fall into the canals already mentioned, and which, 

 drawn outwards by the current by which the sponge is 

 constantly traversed, constitute species of larvae or reproduc- 

 tive bodies, endowed with the locomotive faculty mentioned 

 above. 



A great number of these sponges, or spongiaria, are known 

 to naturalists ; most of them belong to the seas of warm 

 regions, but several live on the rocks of our coast. Those 

 used so abundantly in domestic economy are distinguished by 

 the purely horny nature and by the elasticity of the filaments 

 of which their solid framework is composed ; one of the 

 species, the common sponge, is found in great abundance in 

 the Mediterranean ; another, called usual, belongs to the 

 American seas. These bodies are the object of an important 

 commerce, and to prepare them for the uses to which they are 

 destined it is sufficient to wash them well, so as to detach from 

 their horny skeleton the animal matter with which it is 

 naturally covered. 



OF THE GEOGRAPHICAL DISTRIBUTION 

 OF ANIMALS. 



624. To form a general idea of the animal kingdom, it 

 is not sufficient merely to know the principal phenomena by 

 which life manifests itself in animated beings, and to have 

 studied the structure of their bodies and the mechanism of 

 their functions ; it is also necessary to take a comprehensive 

 and general view of the manner in which animals are spread 

 over the surface of the globe, and to endeavour to appreciate 

 the influence exercised, or which may be exercised, over them 

 by the various circumstances in the midst of which they are 

 destined to live. 



625. When we direct our attention to the manner in 

 which animals are distributed around us on the globe, we are 

 at first struck with the difference of the media in which they 

 live. Some, as every one knows, live always under the 

 waters, and die speedily when they are removed from this 

 liquid ; others can live only in air, and perish so soon as they 

 are immerged. Some, in fact, are destined to people the 

 waters, others to live on land ; and when we compare, physio- 



