66 . THE PHYSIOLOGY OF THE COMMON CRAYFISH. 
three lobes, one anterior, one lateral, and one posterior ; 
and each lobe has its main duct, into which all the tubes 
composing it open. The three ducts unite together into 
a wide common duct (bd), which opens, just behind the py- 
loric valves, into the floor of the mid-gut. Hence the aper- 
tures of the two hepatic ducts are seen, one on each side, 
in this part of the alimentary canal when it is laid open 
from above. Every cecum of the liver has a thin outer 
wall, lined internally by a layer of cells, constituting what 
is termed an epithelium; and, at the openings of the 
hepatic ducts, this epithelium passes into a layer of some- 
what similar structure, which lines the mid-gut, and is 
continued ‘through the rest of the alimentary canal, 
beneath the cuticula. Hence the liver may be regarded 
as a much divided side pouch of the mid-gut. 
The epithelium is made up of nucleated cells, which are 
particles of simple living matter, or protoplasm, in the 
midst of each of which is a rounded body, which is termed 
the nucleus. It is these cells which are the seat of the 
manufacturing process which results in the formation of 
the secretion ; it is, as it were, their special business to 
form that secretion. To this end they are constantly being 
newly formed at the summits of the ceca. As they grow, 
they pass down towards the duct and, at the same time, 
separate into their interior certain special products, 
among which globules of yellow fatty matter are very 
conspicuous. When these products are fully formed, what: 
remains of the substance of the cells dissolves away, and 
