which is formed by a simple lip, but the post-oral
rim is composed of a closely set row of tentacles. These may number some
thousands, and they are usually bent over and tend to form a closed
cylinder of the gutter. Each of these tentacles (fig. 22) is hollow, and
it contains a diverticulum from the coelom, a branch of the vascular
system, a nerve and some muscle-fibres. Externally on two sides and on
the inner surface the tentacles are ciliated, and the cilia are
continued across the gutter to the lip and even on the outer surface of
the latter. These cilia pass on any diatoms and other minute organism
which come within their range of action to the capacious oval mouth,
which appears as a mere deepening of the gutter in the middle line. In
_Terebratulina, Rhynchonella, Lingula_, and possibly other genera, the
arms can be unrolled and protruded from the opened shell; in this case
the tentacles also straighten themselves and wave about in the water.

_The Body Cavity._--The various internal organs of the brachiopod
body, the alimentary canal and liver, the excretory organs, the heart,
numerous muscles and the reproductive organs, are enclosed in a cavity
called the body cavity, and since this cavity (i.) is derived from the
archicoel and is from the first surrounded by meroblast, (ii.)
communicates with the exterior through the nephridia or excretory
organs, and (iii.) gives rise by the proliferation of the cells which
line it to the ova and spermatoza, it is of the nature of a true
coelom. The coelom then is a spacious chamber surrounding the
alimentary canal, and is continued dorsally and ventrally into the
sinuses of the mantle (fig. 21). Some of the endothelial cells lining
the coelom are ciliated, the cilia keeping the corpusculated fluid
contents in movement. Others of the endothelial cells show a great
tendency to form muscle fibres. Besides this main coelomic cavity
there are certain other spaces which F. Blochmann regards as coelomic,
but it must be remembered that his interpretation rests largely on
histological grounds, and at present embryological confirmation is
wanting. These spaces are as follows:--(i.) the great arm-sinus; (ii.)
the small arm-sinus together with the central sinus and the
peri-oesophageal sinus, and in _Discinisca_ and _Lingula_, and, to a
less extent, in _Crania_, the lip-sinus; (iii.) certain portions of
the general body cavity which in _Crania_