coelom. The stalk is an extension of the ventral
body-wall, and contains a portion of the coelom which, in _Discinisca_
and _Lingula_, remains in communication with the general body cavity.

[Illustration: FIG. 22.--Diagrammatic section through an arm of the
lophophore of _Crania_. Magnified; after Blochmann.

1. The lip.
2. The base of a tentacle bisected in the middle line.
3. Great arm-sinus.
4. Small arm-sinus, containing muscle-fibres.
5. Tentacular canal.
6. External tentacular muscle.
7. Tentacular blood-vessel arising from the cut arm-vessel in the
small arm-sinus.
8. Chief arm-nerve.
9. Secondary arm-nerve.
10. Under arm-nerve.]

_The Alimentary Canal_.--The mouth, which is quite devoid of armature,
leads imperceptibly into a short and dorsally directed oesophagus. The
latter enlarges into a spherical stomach into which open the broad
ducts of the so-called liver. The stomach then passes into an
intestine, which in the Testicardines (Articulata) is short,
finger-shaped and closed, and in the Ecardines (Inarticulata) is
longer, turned back upon its first course, and ends in an anus. In
_Lingula_ and _Discina_ the anus lies to the right in the
mantle-cavity, but in _Crania_ it opens medianly into a posterior
extension of the same. Apart from the asymmetry of the intestine
caused by the lateral position of the anus in the two genera just
named, Brachiopods are bilaterally symmetrical animals.

The liver consists of a right and left half, each opening by a broad
duct into the stomach. Each half consists of many lobes which may
branch, and the whole takes up a considerable proportion of the space
in the body cavity. The food passes into these lobes, which may be
found crowded with diatoms, and without doubt a large part of the
digestion is carried on inside the liver. The stomach, oesophagus and
intestine are ciliated on their inner surface. The intestine is slung
by a median dorsal and ventral mesentery which divides the body cavity
into two symmetrically shaped halves; it is "stayed" by two transverse
septa, the anterior or gastroparietal band running from the stomach to
the body wall and the posterior or ileoparietal band running from the
intestine to the body wall. None of these septa is complete, and the
various parts of the central body cavity freely communicate with one
another. In _R