This part of vertebrate organization
always exhibits a differentiation into mouth, pharynx,
oesophagus, stomach, and intestine; and the last has always
a median, or nearly median, aperture on the ventral surface
of the body. It may open by itself; or into a cloaca
, or chamber
common to it, the urinary and the genital organs.
The intestine is generally distinguishable into small
; and, at the junction of the two, one or two coeca are
frequently developed from the former.
The stomach and intestine are invested by a peritoneal
membrane, and connected, by mesogastric
of that membrane, with the median dorsal wall of the abdominal
cavity. Glands appertaining to the lymphatic system
frequently abound in the mesenteric folds, and a highly-vascular
gland of this system, the spleen
, is always (except in Amphioxus, Myxine
, and the Leptocephalidae
) developed in
close proximity to the stomach. A pancreatic
generally pours its secretion into the anterior end of the intestine. Salivary glands
very commonly open into the mouth;
and, in the higher Vertebrata, anal glands
are not unusually
developed in connection with the termination of the rectum.
The structures connected with the alimentary canal of vertebrate
animals, which are most characteristic and peculiar,
are the liver and the teeth.