ALIMENTARY SYSTEM — DIGESTIVE GLAND. 
279 
preventing the laceration of the walls of the stomach and intestines, 
and facilitating the ])assage from the body of the irritant objects. 
These protective developments are, however, not persistent organs, 
but disappear and are renewed periodically, probably forming during 
times of plentiful food supply, and disappearing when a prolonged 
scarcity of food has led to their ab.sorption by the animal, in a similar 
way to that in which other animals absorb superabnndant tissues 
during periods of famine. 
The Digestive Gland, perhaps better known as the liver, is the 
active organ of digestion and is a large brownish or greenish organ, 
partially or entirely enveloping the stomach and adjacent organs, 
more especially in the Peleeypoda, in which it is more developed than 
in the Gastropoda, an increased size of the digestive glands being often 
correlated with a reduction in the extent of the gut. 
The organ is usually formed by two chief, but unequal, lobes, the 
left lobe being larger than the right, this asymmetry being apparent 
from the first development or rapidly 
becoming so, except in Neritlna and 
Valmta, in which they are e(|ual and 
symmetrical during development. It 
consists of innumerable acini, cleft 
into digitiform processes or cceca, 
which are aggregated into lobules of 
varied size and complexity, bound to- 
gether and surrounded by connective 
tissue and usually encompassed by a 
plexus of delicate blood vessels, which 
in Avion ater and other species are 
opaque milk-white showing strikingly 
against the dark background, while 
externally the whole gland is sur- 
rounded by a blond sinus. There is 
an outer structureless membrane, and 
internally the organ is chietly com- 
posed of three kinds of cells, viz., fer- 
ment cells, liver cells and lime cells. 
The ferment and liver cells enclose 
grannies of fat, albumen and pigment and differ chiefly physiologically. 
The lime cells, which contain phosphate or carbonate of lime, are 
Fig. .550. — Digestive tract of Lwimca 
stagnatisiX'^^’, X 2, showing the digestive 
gland and its ducts. 
b.b. buccal bulb ; s.g, salivary glands ; 
O’, oe.sophagus ; c, crop ; g. gizzards ; 
st. stomach : d.gl, digestive gland and 
its ducts ; anus. 
