BLOOD SYSTEM. 



409 



ings on the sides of the heart by which blood may be 

 taken in. The diagram (Fig. 287) is an attempt to rep- 

 resent schematically the general course of the circula- 

 tion. The dotted lines represent the lacunary circula- 

 tion without definite vessels. 



The difference between this lacunary circulation and 

 a true vascular circulation may be illustrated by an irri- 

 gation system. The circula- 

 tion of vertebrates may be 

 compared to a pump and a sys- 

 tem of pipes closed through- 

 out and impermeable until the 

 soil to be irrigated is reached 

 and there permeable. After 

 use the overplus of liquid is 

 again gathered into imperme- 

 able pipes and returned to the 

 pump to be again used. The 

 circulatory system of an in- 

 sect, on the contrary, is like 

 a pump with short pipes dis- 

 charging on the soil in front 

 and sucking up from the soil 

 behind. But the soil, instead 

 of being penetrated in all di- 

 rections by pipes which con- 

 fine and guide the currents, is 

 covered with little pools connected by channels. Under 

 these conditions the water would work around in an in- 

 definite way, and be sucked up behind, to be used again. 



Respiratory System. — Now, it is this peculiar 

 mode of circulation that compels the very exceptional 

 kind of respiratory apparatus. Insects are active, and 

 somewhat warm-blooded animals, and therefore require 

 a perfect aeration of the blood ; but with a lacunary cir- 



FlG. 288. — Tracheal system of an 

 insect : sp sp, spiracles ; s s, 

 air sacs. 



