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Sipuncula
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{{Taxobox
| color = pink
| name = Peanut worms
| regnum =
Animalia
| subregnum =
Metazoa
| superphylum =
Lophotrochozoa
| phylum = '''Sipuncula'''
| phylum_authority =
Constantine Samuel Rafinesque-Schmaltz Rafinesque, 1814
| subdivision_ranks =
Class (biology) Classes,
Order (biology) Orders and
Family (biology) Families
| subdivision =
*Class
Sipunculidea
**Order
Sipunculiformes
***Family
Sipunculidae
**Order
Golfingiiformes
***Family
Golfingiidae
***Family
Phascolionidae
***Family
Themistidae
*Class
Phascolosomatidea
**Order
Phascolosomatiformes
***Family
Phascolosomatidae
**Order
Aspidosiphoniformes
***Family
Aspidosiphonidae
}}
The '''Sipuncula''', sipunculid worms or '''peanut worms''', are a
Phylum (biology) phylum containing 144-320
species (estimates vary) of bilaterally
symmetry symmetrical,
segmentation (biology) unsegmented sea marine worms. '''Sipunculid worm jelly''' (土笋冻) is a delicacy in the town of
Xiamen in
Fujian province of
China.
Habitat
Sipunculids are relatively common, and live in shallow waters, either in
burrows or in discarded
animal shell shells like
hermit crabs do. Some bore into solid rocks to make a shelter for themselves. Although typically less than 10
centimetre cm long, some sipunculans may reach several times that length.
Anatomy
The most noticeable feature of sipunculan worms is their mouth, which is surrounded by a ring of
tentacles, all of which may be inverted into the body. The
digestive tract of sipunculans passes from the mouth to the posterior end of the body, before twisting back around itself and ending at the
anus, on the side of the body. A few taxa possess a calcified plate called the anal shield. Sipunculans have a
body cavity coelom. However, they do not have a vascular blood system. Instead, interstitial fluid transports
oxygen and
nutrients around the body. A separate cavity fills the hollow tentacles; it passes oxygen from the tentacles to the coelom. The body wall is strong and
muscle muscular; when threatened, sipunculids can retract their body into a shape resembling a
peanut seed kernel. This is where the name "peanut worm" comes from.
Relationships
The
phylogenetics phylogenetic placement of this phylum has proved troublesome. Originally classified as
annelids, despite the complete lack of segmentation,
bristles and other annelid characters, the phylum Sipuncula was later allied with the
Mollusca, mostly from
developmental and
larval characters. Nowadays, these two phyla are generally included in a larger group, the
Lophotrochozoa, that also includes
annelids,
nemertea ribbon worms, and four other phyla.
Fossil record
The fossil record of the Sipuncula is, not surprisingly for soft-bodied animals, sparse. Some scientists, however, consider that
hyoliths,
Operculum (gastropod) operculate shells from the
Palaeozoic may be related to the sipunculids, with the only remnant of the shell in extant forms being the anal plate.
External links
-
Sipuncula taxa with synonyms
Category:Animals
de:Spritzwürmer
it:Sipuncula
pl:Sikwiaki
pt:Sipuncula
ru:Сипункулиды
fi:Ruiskumadot
zh:星虫动物门
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