Organs of Circulation. 67 



and receives the blood at valvular openings (Fig. 19, o) 

 along its sides which only permit the fluid to pass in, when 

 by contraction it is forced toward the head and emptied 

 into the general cavity. Thus the heart only serves to 

 keep the blood in motion. 



There are no vessels to carry the blood to the various 

 organs, nor is this necessary, for the nutritive fluid every- 

 where bathes the digestive canal, and thus easily receives 



Fig. iq. 



Portion oj Henri of an Insect, ajler Paclard. 



li Heart 

 m Muscles, 

 o Openings. 



nutriment, or gives waste by osmosis; everywhere sur- 

 rounds the trachese or air-tubes — the insect's lungs — and 

 thus receives that most needful of all food, oxygen, and 

 gives the baneful carbonic acid; everywhere touches the 

 various organs, and gives and takes as the vital operations 

 of the animal require. 



The heart, like animal vessels, generally, consists of an 

 outer serous membrane, an inner, epithelial coat, and a 

 middle muscular layer. Owing to the opaque crust, the 

 pulsations of the heart cannot generally ne seen; but in 

 some transparent larvae, like man}' maggots, some parasites 

 — those of our common cabbage butterfly show this admir- 

 ably — and especially in aquatic larv£E, the pulsations are 

 plainly visible and are most interesting objects of study. 



The heart, as shown by Lyonnet, is held to the dorsal 

 wall by muscles (Fig. 19, ni). Beneath the heart are mus- 

 cles which, to quote from Girard, form a sort of horizontal 

 diaphragm (Fig. 20, <^), which as Graber shows contract 

 and thus aid circulation. 



