DISTINGUISHING CHARACTERS OF INSECTS. 19 



tion, are without cessation supplied \vith material for 

 their mysterious operations. 



Through the arteries this stream of life, impelled by 

 the repeated action of the heart, continues to pour ; but 

 at each point a portion of the stream, deprived of its 

 oxygen and of other component parts, becomes debased 

 and loses its value and power : by another series of 

 channels therefore, the veins, it flows back towards the 

 heart, receiving from the stomach on its way fresh nutri- 

 tive material, again is sent to the lungs to gather from 

 the air contained in them the oxygen by whiph its life- 

 giving powers are renewed, and again is propelled into 

 the arteries to recommence the circuit of the body. 



Now, leaving untouched all other anatomical details 

 and physiological phenomena, let us compare what has 

 been said of the vertebrata with the facts which we find 

 in the invertebrata, using in this comparison, in order to 

 shorten and simplify the chapter, only the order of insects 

 from among the invertebrata in which this order forms 

 a perfect group. 



Insects are without any internal skeleton at all. The 

 body is supported by an external more or less hard and 

 jointed case, which forms the covering of the body, and 

 to which the muscles are attached, as in the vertebrata 

 to the internal skeleton. This case, in fact, answers the 

 purposes of both skin and skeleton. 



The limbs of a perfectly-developed insect consist of 

 six jointed legs, neither less nor more; certain four- 

 legged butterflies being merely instances of aborted 

 limbs, while in the many-legged caterpillars the extra 

 "legs," as they are called, are sucker-like and jointless 

 processes of the skin. 



As in the vertebrata, so in insects, there is great 



