the long, white alimentary canal, crowded with mud. The mouth is
devoid of armature, and passes without break into the oesophagus;
this is surrounded by the retractor muscles, which are inserted
into the skin around the mouth, and have their origin in the
body-wall, usually about one-third or one-half of the body-length from the
anterior end (figs. 1 and 2). Their function is to retract the introvert,
which is protruded again by the contraction of the circular muscles
of the skin; these, compressing the fluid of the body-cavity, force
forward the anterior edge of
the introvert. The number
of muscles varies from one
(Onchnesoma and Tylosoma)
to four, the latter being very
common. The alimentary
canal is U-shaped, the dorsal
limb of the U terminating in
the anus, situated not very
far from the level of the
origin of the retractor
muscles. The limbs of the
U are further twisted
together in a looser or tighter
coil, the axis of which may
be traversed by a “spindle”
muscle arising from the
posterior end of the body. No
glands open into the
alimentary canal, but a
diverticulum, which varies
enormously in size, opens into
the rectum. As is so often
the case with animals which
eat mud and sand, and
extract what little nutriment
is afforded by the
organic débris therein, the
walls of the alimentary
canal are thin and apparently
weak. All along one
side is a microscopic ciliated
groove, into which the mud
does not seem to enter, and
along which a continuous
stream of water may be
kept up. Possibly this is
respiratory—there are no
special respiratory organs.
A so-called heart lies on the
dorsal surface of the
oesophagus; it is closed behind,
but in front it opens into a
circumoesophageal ring,
which gives off vessels into
the lophophore and
tentacles. The contraction of
this heart, which is not
rhythmic, brings about the
expansion of the tentacles
and lophophore. This system
is in no true sense a
vascular system; there are
no capillaries, and the fluid
it contains, which is
corpusculated, can hardly have
a respiratory or nutritive
function. It is simply a
hydrostatic mechanism for
expanding the tentacles.
The excretory organs are
typical nephridia, with an
internal ciliated opening
into the body-cavity, and
an external pore. One
surface of the tube is
prolonged into a large sac lined
with glandular excretory
cells. The organs are
typically two, though one is
often absent, e.g. in
Phascolion. They serve as
channels by which the
reproductive cells leave the
body, and they are
sometimes spoken of as “brown
tubes.” There is a
well-developed brain dorsal to
the mouth; this gives off a pair of oesophageal commissures,
which surround the oesophagus and unite in a median ventral
nerve-cord which runs between the longitudinal muscles to the
posterior end of the body. From time to time it gives off
minute circular nerves, which run round the body in the skin
and break up into a very fine nerve plexus. There are no distinct
ganglia, but ganglion cells are uniformly distributed along the
ventral side of the cord. The whole is anteriorly somewhat loosely
slung to the skin, so as to allow free play when the animal is extending
or retracting its introvert. A pit or depression, known as “the
cerebral organ,” opens into the brain just above the mouth; this
usually divides into two limbs, which are deeply pigmented and
have been called eyes.
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Fig. 1.—Sipunculus nudus, L., with introvert and head fully extended, laid open by an incision along the right side to show the internal organs. | ||||||||||||||||||||
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Sipunculoids are dioecious, and the ova and spermatozoa are formed from the modified cells lining the body-cavity, which are heaped up into a low ridge running along the line of origin of the retractor muscles. The ova and the mother-cells of the spermatozoa break off from this ridge, and increase in size considerably in the fluid of the body-cavity. Fertilization is external; and in about three days a small ciliated larva, not unlike that of the Echiuroids, but with no trace of segmentation, emerges from the egg-shell. This little creature, which has many of the features of a Trochosphere larva, swims about at the surface of the sea for about a month and grows rapidly. At the end of this time it undergoes a rapid metamorphosis: it loses many of its larval organs, cilia, takes in a quantity of water into its body-cavity, sinks to the bottom of the sea, and begins life in its final form.
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Fig. 2.—Right half of the anterior end of Sipunculus nudus, L., seen from the inner side and magnified. | ||||||||||||||||
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The following genera of Sipunculoids are recognized:—(i.) Sipunculus. This, with Physcosoma, has its longitudinal muscles divided up into some 17-41 bundles. It has no skin papillae. The members of this genus attain a larger size than any other species, and the genus contains some 16-17 species. (ii.) Physcosoma (fig. 3) has its body covered with papillae, and usually numerous rows of minute hooks encircling the introvert. It is the most numerous genus, and consists for the most part of shallow-water (less than 50 fathoms) tropical and subtropical forms. They often live in tubular burrowings in coral-rock. The following three genera have their longitudinal muscles in a continuous sheath:—(iii.) Phascolosoma, with some 25 species, mostly small, with numerous tentacles. (iv.) Phascolion, 10 species, small, living in mollusc-shells and usually adopting the coiled shape of their house; only one kidney, the right, persists. (v.) Dendrostoma, with 4-6 tentacles, a small genus found in tropical shallow water. (vi.) Aspidosiphon, with 19 species, is easily distinguished by a calcareous deposit and thickened shield at the posterior end and at the base of the introvert, which is eccentric. (vii.) Cloeosiphon has a calcareous ring, made up of lozenge-shaped plates, round the base of its centric introvert. (viii.) Petalostoma, a
