ESSENTIAL STRUCTURE OF SECRETING GLANDS. 301 
Crustacea, by referring to fig. 47, fo ; and in the Mollusca it is 
nearly the same (figs. 157, l, and 149,/). 
357. The required extent of secreting surface is not unfre- 
quently given, however, by the prolongation of the follicles 
into tubes, rather than by a great multiplication in their 
number. Of this we have a remarkable example in the 
Kidney of the higher animals (§ 368), which is entirely com¬ 
posed of such tubes, together with areolar tissue which binds 
them together, and the blood-vessels distributed amongst them. 
These tubes, like the follicles, are lined with epithelium-cells 
(fig. 166), which are the real instruments in the separation of 
their secreted product. 
358. That there is nothing in the form of any secreting 
apparatus, however, which determines the peculiar nature of 
its secretion, is evident from this 
fact,—that, in glancing through the 
Animal series, we find the same secre¬ 
tion elaborated by glandular struc¬ 
tures of every variety of form. Thus, 
we have seen that the bile is secreted, 
in the lowest animals in which we 
can distinguish it, by a number of 
distinct follicles, as simple in their 
structure as are those by which the 
mucous secretions are formed in the 
highest. Again, the bile is secreted 
in Insects, by a small number of long 
tubes, which open separately into the 
intestinal canal just below the stomach 
(fig. 112); and these tubes appa¬ 
rently differ in no respect from those 
that form the urinary secretion in the 
same animals, which open nearer the 
outlet of the intestinal canal. In 
fact, the distinct function of the 
latter was not known, until it was 
ascertained that uric acid is to be 
found in them. In fig. 167, which 
represents the digestive apparatus of the Cockchafer, it is 
seen that the biliary vessels are only four in number, but 
are very long ; and that, for a good part of their length, 
Fig. 167. 
Alimentary Canal and 
HEPATIC TUBULES OF COCK¬ 
CHAFER. 
