Organs of Circulation. 67 
and receives the blood at valvular openings (Fig. 19, 0) 
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 
Portion of Heart of an Insect, after Packard, 
Hf Heart. 
m Muscles, 
& Openings, 
nutriment, or gives waste by osmosis; everywhere sur- 
rounds the trachee 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 ati 
outer serous membrane, an inner, epithelial coat, and a 
middle muscular layer. Owing to the opaque crust, the 
pulsations of the heart cannot generally pe seen; but in 
some transparent larve, like many maggots, some parasites 
—those of our common cabbage butterfly show this admir- 
ably—and especially in aquatic larva, 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 museles (Fig. 19,7). Beneath the heart are mus- 
cles which, to quote from Girard, form a sort of horizontal 
diaphragm (Fig. 20, d ), which as Graber shows contract 
and thus aid circulation. 
