Themiste, a sipunculan.
Phylum Sipuncula (sigh-pun´kyu-la)
, little siphon) consists of
benthic marine worms, predominantly
littoral or sublittoral. They live sedentary
lives in burrows in mud or sand,
occupy borrowed snail shells, or live in
coral crevices or among vegetation.
Some species construct their own rock
burrows by chemical and perhaps
mechanical means. More than half the
species are restricted to tropical zones.
Some are tiny, slender worms, but the
majority range from 15 to 30 cm in
length. Some are commonly known as
“peanut worms” because, when disturbed,
they can contract to a peanut
shape (Figure 21-1).
Sipunculans have no segmentation
or setae. They are most easily recognized
by a slender retractile introvert
, which is continually
and rapidly being run in and out of the
anterior end. Walls of the trunk
muscular. When the introvert is
everted, the mouth can be seen at its
tip surrounded by a crown of ciliated
tentacles. Undisturbed sipunculans
usually extend the anterior end from
the burrow or hiding place and stretch
out their tentacles to explore and feed.
They are largely deposit feeders living
on organic matter collected in mucus on the tentacles and moved to the
mouth by ciliary action. The introvert
is extended by hydrostatic pressure
produced by contraction of the bodywall
muscles against the coelomic
fluid. The lumen of the hollow tentacles
is not connected to the coelom but
rather to one or two blind, tubular
compensation sacs that lie along the
esophagus (Figure 21-2). The sacs
receive fluid from the tentacles when
the introvert is retracted. Retraction is
effected by special retractor muscles.
The surface of the introvert is often
rough because of surface spines,
hooks, or papillae.
Internal structure of Sipunculus
There is a large, fluid-filled coelom
traversed by muscle and connective
tissue fibers. The digestive tract is a
long tube that doubles back on itself to
end in the anus near the base of the
introvert (Figure 21-2). A pair of large
nephridia opens to the outside to
expel waste-filled coelomic amebocytes;
the nephridia also serve as gonoducts.
Circulatory and respiratory systems
are lacking, but the coelomic
fluid contains red corpuscles that contain
a respiratory pigment, hemerythrin,
used in transportation of oxygen.
The nervous system has a bilobed
cerebral ganglion just behind the tentacles
and a ventral nerve cord extending
the length of the body. Sexes are separate. Permanent gonads are lacking,
and ovaries or testes develop seasonally
in the connective tissue covering
the origins of one or more of the
retractor muscles. Sex cells are
released through the nephridia. The
larval form is usually a trochophore.
Asexual reproduction also occurs by
transverse fission, the posterior onefifth
of the parent constricting off to
become a new individual.
There are approximately 330 species
and 16 genera, which are placed by
some authorities into four families. The
best-known genera are probably Sipunculus,
leather bag, pouch, + soma, body),
Aspidosiphon (Gr. aspidos, shield,
, siphon), and Golfingia
(named by E. R. Lankester in honor of
an afternoon of golfing at St. Andrews,