268 THE VITAL FUNCTIONS. 



blood from the system, unite to form one or 

 more vessels, which then assume the function 

 of arteries, subdividing and ramifying upon the 

 branchial organs ; after this the blood is again 

 collected by the branchial veins, which unite 

 into one trunk to form the arteries of the sys- 

 temic circulation. 



Most insects, especially when arrived at the 

 advanced stages of their developement, have too 

 imperfect a circulation to effect the thorough 

 aeration of the blood : and indeed the greater part 

 of that fluid is not contained within the vascular 

 system, but permeates the cavities and cellular 

 texture of the body. It will be seen, when I 

 come to treat of respiration, that the same object 

 is accomplished by means totally independent of 

 the circulatory apparatus ; namely, by a system 

 of air-tubes, distributed over every part of the 

 body. But an apparatus of this kind is not 

 required in those Arachnida where the circulation 

 is vigorous, and continues during the whole of 

 life: here, then, we again meet with a pulmonary 

 as well as a systemic circulation, in conjunction 

 with internal cavities for the reception of air. 



In the Crustacea the circulation is conducted 

 on the same general plan as in the Annelida ; the 

 blood from every part of the body being collected 

 by the Venai Cavae, which are exceedingly capa- 

 cious, and extend, on each side, along the lower 

 surface of the abdomen. They send out branches, 



