122 COMPARATIVE ZOOLOGY. 
the tubular follicles. The follicles are less than 3,5 of 
an inch in diameter, and number millions. 
3. The pancreas, or “sweetbread,” so important in the 
process of digestion, when present, exists only in the Ver- 
tebrates and the higher 
Mollusks. In its struct- 
ure and its secretion it 
closely resembles the sal- 
ivary glands. In the Cut- 
tle-fish, it is represented 
by a sac; in Fishes, by 
a group of follicles. It 
is proportionally largest 
in Birds whose salivary 
glands are deficient. The 
Fre. 91.—Pancreas of Man, 0; g, gall-bladder ; $ ee P 
8, cystic duct; c, duct from the liver; p, py- pancreatic juice enters 
loric valve; e, 7, duodenum. ; 
the duodenum. 
4, A liver in some form is found in all animals having 
a distinct digestive cavity. In Mollusks and Vertebrates, 
it is the largest gland in the body. The higher the ani- 
mal, the more compact the organ. Thus, in Polyps it is 
represented by yellowish cells lining the stomach; in In- 
sects, by delicate tubes along the intestine; in Mollusks, 
by a cluster of sacs, or follicles, forming a loose compound 
gland. In Vertebrates, the liver is well defined, and com- 
posed of a multitude of lobules (which give it a granular 
appearance) arranged on the capillary veins, like grapes 
on a stem, and containing nucleated secreting cells. It 
is of variable shape, but usually two, three, or five lobed, 
and is centrally situated—in Mammals, just below the 
diaphragm. In most Vertebrates, there is an appendage 
to the liver, called the gall-bladder, which is simply a res- 
ervoir for the bile when not wanted. 
The liver is both a secretory and excretory organ. For 
while the bile performs an essential, though mysterious, 
