INTERNAL ANATOMY OF INSECTS. 103 
other Classes, often an index of a less substantial kind 
of nutriment ; while their shortness and slenderness in- 
dicate that the insect lives by prey\ 
In considering therefore the parts connected with the 
digestive functions of the insect world, it will not be amiss 
to have reference to their Jbod, and their mode of taking 
it; but first it will be proper to state and define the parts 
of this important organ. 
In general the alimentary canal b is composed of the 
same essential tunicki as that of the vertebrate animals, 
consisting of an interior epidermis, apapillaryand cellular 
tunick, and an exterior muscular one e . The first is 
usually tender, smooth, and transparent; but not always 
discoverable, perhaps on account of its tender sub- 
stance d . Iiamdohr does not notice the papillary and 
cellular tunicks ; they are probably synonymous with 
what he denominates — Xhejlochy layer (Diejlockige lage\ 
and which he describes, when highly magnified, as ap- 
pearing to consist of very minute globules or dark points, 
and as being of a cellular structure e . The exterior tu- 
nick is thicker and stronger than the interior, and com- 
posed of muscular fibres, running either longitudinally, 
or transversely so as to form rings round the canal. 
This tunick mostly begins at the mouth, and goes to the 
anus, changing its conformation in different parts of the 
above intestine. Sometimes however it originates only 
at the beginning of the stomach f . With respect to its 
general disposition, that canal — in its relative length, in 
n Cuv. Anat. Comp. iv. 129. 
b Plate XXI. Fig. 3. c, d, e, is the intestinal canal of the larva 
of the Cossus. c Cuv. Ibid. 112. 
" Ramdohr Anat. der Ins. 6. € Ibid. 25. ' Ibid. 6, 
