BLOOD SYSTEM. , Q y 



This kind of aeration is sufficient for animals so low 

 in the scale of life, and especially of so small size, for in 

 these small animals the surface is large in proportion 

 to the bulk. But with increasing size, since bulk in- 

 creases as the cube while surface only as the square of 

 diameter, it is evident that the same degree of aeration 

 can not be effected without some device to increase 

 the surface of contact — i. e., a respiratory organ; and 

 this must be still further increased when the organiza- 

 tion is higher as well as the bulk greater. And then, 

 last of all, for greater efficiency the whole is relegated 

 to an internal surface. 



The same is true of the nutritive system proper — i. e., 

 food system and blood system. In a small body suf- 

 ficient food may be taken directly by a simple surface, 

 external or internal ; but in a large body the absorbing 

 surface is not great enough, and many parts are too far 

 away from the absorbing surface. There must be a sys- 

 tem of vessels to carry nutriment to distant parts. This 

 is the blood system. To illustrate : In a small island no 

 elaborate system of internal carrying trade is necessary, 

 for all parts are near the coast where products are 

 delivered. But as the island becomes larger, and espe- 

 cially as the commercial life becomes higher, the inter- 

 nal carrying trade becomes more elaborate. 



INSECTS. 



It will be remembered that we passed over these be- 

 cause the whole plan of their circulation and respiration 

 is entirely different and wholly out of the line of gradual 

 simplification which we otherwise find. In all other 

 classes the respiration controls the course of the blood, 

 and was taken up first ; but in insects the blood system 

 controls the character of the air system, and therefore 

 the blood system must be taken up first. 



