INTERNAL ANATOMY OF INSECTS. 89 
those animals that have a circulation, this takes place by 
means of lungs or gills; — thus we find, even in the Crus- 
tacea and Arachnida so nearly related to insects, that 
the organs of this function are true gills ,« whereas in in- 
sects, though in some of their states their respiratory 
tubes are branchiform, yet they are not gills, and the 
respiration is by tubes and spiracles. And these tubes, 
as you have seen, are so numerous and so infinitely ra- 
mified and dispersed, as to occupy the place of arteries 
and veins, and to imitate their distribution, — and thus 
to oxygenate what may be deemed the real analogue of 
the blood, which bathes every internal part of the body 
of an insect. Those animals likewise that have a circula- 
tion are furnished with a liver, as is the case with the 
Arachnida and even many aggregate animals that have 
a heart; but in insects there are only hepatic ducts. 
M. Cuvier has also proved that the conglomerate glands, 
which exist in all animals that have a heart and blood- 
vessels, do not exist in insects, in which they are replaced 
by long slender secretory tubes, which without being 
united float in the interior of the body: from this circum- 
stance, he is led to conclude that their nutrition is by 
imbibition or immediate absorption, as in the Polypi send 
other zoophytes, the chyle transpiring through the ali- 
mentary canal, and running uniformly to all parts of 
the body a . 
These arguments appear so satisfactory, that Physio- 
logists in general seem to have been convinced by them 
that no circulation, at any time, takes place in insects, 
and that their supposed heart is merely a secretory ves- 
sel, though of what kind they were at a loss to conjec- 
1 .V. Diet. d'Hist. Nat. xvi. 208. 
