INTERNAL ANATOMY OF INSECTS. Ill 
and when numerous, they are generally attached singly 
though irregularly *. These vessels at their base do 
not open into the cavity of the intestinal canal, but merely 
into the space between its outer and inner tunicks, the 
last being constantly imperforated b . 
With regard to their apex, the bile-vessels are some- 
times fxed singly or connectedly ta the intestine merely 
by a few muscular fibres ; for they do not enter it, their 
ends having no orifice. This structure is mostly to be met 
with in the Coleoptera c . In caterpillars, the tops of these 
vessels perforate the outer skin of the rectum, and pro- 
ceeding in dense convolutions to the anus, become at last 
so fine that their terminations cannot be discovered* 1 . 
In other cases, the extremities of a pair of these vessels 
unite so as to form a double one: this may be seen in those 
of Philonthus politus e , and probably other rove-beetles : 
and lastly, in others the bile- vessels are free, hanging 
down by the intestinal canal, without being attached to it 
or to each other. This structure is constantly found 
in the Orthoptera and Hymenoptera Orders, &c. f . 
With regard to their number, the bile-vessels vary from 
two to upwards of one hundred and fifty, yet so that 
their whole amount is constantly the product of the num- 
ber two, — at least as far as they have been counted : and 
even when those on one side are not alike, a similar va- 
riation takes place in the other, as may be seen in Gal- 
leruca Vitellines, where on each side are two long ones 
and one shorter s ; the most usual numbers are, four — 
six — or many, that is, more than twenty — 
a Ramdohr,*. xiii./. 1—3. b Ibid. 44. 
• Ibid. 45. d Ibid. 45. Plate XXI. Fig. 3.f.f. 
• Rhamdohr, Ibid. t. m.f. 6. E. 
' Ibid. t. If. 1. 5. 9. t. x\v.f. 1—3. * Ibid. 40. t. vi./. 3. 
