AND DEVELOPMENT OF THE LIVER. 
115 
As reasons for considering- the tissue now described as part of the hepatic appa- 
ratus, I may mention, — 1st, the close resemblance which often exists between the con- 
tents of the vesicles and those of the undoubted biliary tubes ; in several instances, in 
fact, they have appeared to me identical. 2nd. The presence of distinctly yellow 
fluid, either infiltrating- the g-raniilar contents of the vesicles, or united to oil-drops : 
this fluid may be reasonably supposed to be of the nature of bile. 3rd. The existence 
of much g-ranular matter, shown by chemical tests to be of albuminous nature, of 
granular cells and nuclei in the vesicles, shows that they cannot be regarded as 
adipose tissue merely; nor should we expect to find this in such constancy and abun- 
dance in a class no higher than that of Insects. 4th. The oily matter which is found 
in them is usually not at all more abundant than in the liver of many higher animals ; 
it exists also in a state of diffusion and combination with the granular matter, and 
its presence (since the tissue is surely not mere adipose) argues in some measure in 
favour of its hepatic character, it being well known how prone oily matter is to 
accumulate in hepatic structure. Were it proved by chemical tests that the yellow 
fluid, not unfrequently seen in the vesicles, is truly bile, no further argument would 
be needed ; of this however I have been unable to obtain positive evidence ; yet once 
I observed the yellow tint to be decidedly deepened by nitric acid, and from the 
action of this reagent and sulphuric acid in other instances, I am pretty well satisfied 
that albumen and oily matter are not the only contents of the vesicles. 
The above arguments, though far from conclusive in favour of the opinion I have 
adopted, are not without weight ; and I will only add, that if this view should prove 
correct, an analogy will obtain between the hepatic apparatus of the Leech and that 
now described as existing in Insects. 
The structure of the liver in Crustaceans is so well known, and has been so well 
described, that I can add nothing to our knowledge respecting it ; I would remark, 
however, that the large quantity of free oily matter which is found in the follicles of 
this class, must be regarded as an indication both of the feeble intensity of the respi- 
ratory process, and also that the secretion is not to be regarded as in any great 
degree excrementitious, since it would not then be found accumulating in such quan- 
tity. Before passing on, however, I must particularly allude to one interesting in- 
dividual which is ranked with this class, the Daphnia monoculus : this little creature, 
from the transparency of its shell, and the simplicity of its organization, is a most 
favourable object for examination. Any one who would study the phenomena of 
muscular action in the living fibre to advantage, should not omit to examine a living 
specimen of this species ; striped fibres exist in the larger ones, and the oscillating 
waves of contraction during the action of the fibre may be distinctly observed. The 
hepatic apparatus in the Daphnia I believe to consist of large cells whose diameter 
is about 7 -loo th of an inch ; these are chiefly aggregated round the intestine, but 
exist also in other parts ; they have a distinct envelope, which encloses a transparent 
fluid and a large reddish yellow oil-drop, with one or two smaller ones ; a nucleus is 
Q 2 
