88 INTERNAL ANATOMY OF INSECTS. 
in the Crustacea and Arachnida so nearly related to mn- 
sects, that the organs of this function are true gills ; 
whereas in insects, 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 
ramified 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 circulation 
are furnished with a Jzver, as is the case with the Arach- 
nida 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 ex- 
ist 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 circumstance, he 
is led to conclude that their nutrition is by imdzbition or 
immediate absorption, as in the Polypz and other zoo- 
phytes, the chyle transpiring through the alimentary ca- 
nal, and running uniformly to all parts of the body, 
But although it be granted that no circulation of the 
blood takes place in insects, yet, reasoning from analogy, 
the dorsal vessel should in some degree and in some re- 
spects represent the heart, and its pulsations be in some 
measure for a similar purpose ; but what that purpose is, 
has not yet been ascertained : and on the whole, in the 
present state of our knowledge, it seems the most prudent 
* N. Dict. a’ Hist. Nat. xvi. 208. 
