Sipunculid , Phascolosoma sp., Modified from Kingsley, 1884.
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Phylum Sipuncula Sipunculid , Phascolosoma sp., Modified from Kingsley, 1884. |
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The "peanut worms" of the phylum Sipuncula
are unsegmented worm-like
organsisms that can usually be found buried in soft sediments (mud or sand), under rocks or in
algal holdfasts
, or burrowing
into calcareous
reefs or other hard substrates, in depths ranging from the
intertidal
to over 5,000 meters deep.
There are approximately 250 living species worldwide, most are less than about 10 cm. long, but a few reach
lengths up to 72 cm. All are marine.
Looking like small sausages or peanuts, they have a retractible "introvert
"
that terminates in a mouth, which is surrounded by tentacles used for feeding.
Cilia
and mucous on
the tentacles are used to trap organic particles, which are drawn into the mouth and gut
when the introvert
is retracted. Some species are indeposit feeders,
ingesting organic particles that have settled on the substrate surface. Burrowing forms
are usually direct deposit feeders,
ingesting the sediment as they burrow (and extracting organic material as it passes through
the gut).
Sipunculids have no trace of segmentation, and may be derived from organisms that diverged from the Annelid line before segmentation arose. They are schizocoelus protostomes , eucoelomate , and have a complete gut (with mouth and anus ). They have no special circulatory system, however the coelomic spaces and fluids (which have red blood cells containing the respiratory pigment hemerythrin ) fulfill basic circulatory functions. References: Barnes, 1980 Barnes, Calow, and Olive, 1993 Brusca & Brusca, 1990 Kingsley, 1884. Meglitsch & Schram, 1991 |
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