are separated off and
contain muscles, &c.; (iv.) the cavity of the stalk when such exists.
The great arm-sinus of each side of the lophophore lies beneath the
fold or lip which together with the tentacles forms the ciliated
groove in which the mouth opens. These sinuses are completely shut off
from all other cavities, they do not open into the main coelomic space
nor into the small arm-sinus, nor does the right sinus communicate
with the left. The small arm-sinus runs along the arms of the
lophophore at the base of the tentacles, and gives off a blind
diverticulum into each of these. This diverticulum contains the
blood-vessel and muscle-fibres (fig. 22). In the region of the mouth
where the two halves of the small arm-sinus approach one another they
open into a central sinus lying beneath the oesophagus and partly
walled in by the two halves of the ventral mesentery. This sinus is
continued round the oesophagus as the peri-oesophageal sinus, and thus
the whole complex of the small arm-sinus has the relations of the
so-called vascular system of a Sipunculid. In _Crania_ it is
completely shut off from the main coelom, but in _Lingula_ it
communicates freely with this cavity. In _Discinisca_ and _Lingula_
there is further a lip-sinus or hollow system of channels which
traverses the supporting tissue of the edge of the mantle and contains
muscle-fibres. It opens into the peri-oesophageal sinus. It is better
developed and more spacious in _Lingula_ than in _Discinisca._ In
_Crania_, where only indications of the lip-sinus occur, there are two
other closed spaces. The posterior occlusor muscles lie in a special
closed space which Blochmann also regards as coelomic. The posterior
end of the intestine is similarly surrounded by a closed coelomic
space known as the peri-anal sinus in which the rectum lies freely,
unsupported by mesenteries. All these spaces contain a similar
coagulable fluid with sparse corpuscles, and all are lined by ciliated
cells. There is further a great tendency for the endothelial cells to
form muscles, and this is especially pronounced in the small
arm-sinus, where a conspicuous muscle is built up. The mantle-sinuses
which form the chief spaces in the mantle are diverticula of the main
coelomic cavity. In _Discinisca_ they are provided with a muscular
valve placed at their point of origin. They contain the same fluid as
the general