266 RAMBLES OF A NATURALIST. 



them, is in itself a great step in advance ; but nature 

 never proceeds by sudden starts, and this perfection 

 is brought about by very gradual steps. When 

 once the digestive cavity is complete, \ve may 

 assume that there will also be present in the animal 

 some liquid specially designed for the maintenance 

 of the organs. Here, too, a preliminary absorption 

 is necessary to enable the materials furnished by 

 digestion to blend with this kind of blood, but still 

 respiration, continues for some time to be effected by 

 the aid of already existing organs, or by the diges- 

 tive apparatus itself. A very large number of An- 

 nelids respire through the skin only, while many 

 Crustaceans have no other branchiaa than their feet. 

 In the larvas of those large insects known as dragon 

 flies, we observe a still more curious phenomenon. 

 Here the intestine presents a considerable dilatation 

 at its posterior extremity. The water penetrates 

 into this cavity, from which it is propelled at the 

 will of the animal. Here, then, we have the respira- 

 tory apparatus. It is easy to ascertain this by hold- 

 ing one of these larvae out of the water, and then, 

 after a time, restoring it to its native element. We 

 shall find that it will draw in and eject the liquid 

 with the same kind of anxious precipitation that is 

 exhibited by a half- suffocated mammal; but while 

 the latter breathes through a mouth, the larva of the 

 dragon-fly respires through the opposite extremity of 

 its alimentary canal. 



The circulation presents an almost infinite variety 

 in the modes of its successive development. Very 

 often it is altogether wanting ; in the lower types 



