From Academic Kids
|Classes, Orders and Families|
The Sipuncula, sipunculid worms or peanut worms, are a phylum containing 144-320 species (estimates vary) of bilaterally symmetrical, unsegmented marine worms. Sipunculid worm jelly (土笋冻) is a delicacy in the town of Xiamen in Fujian province of China.
Sipunculids are relatively common, and live in shallow waters, either in burrows or in discarded shells like hermit crabs do. Some bore into solid rocks to make a shelter for themselves. Although typically less than 10 cm long, some sipunculans may reach several times that length.
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 are coelomate, meaning they possess a fluid-filled cavity inside the body, and this fluid (the coelom) transports oxygen and nutrients around the body. A separate coelomic cavity fills the hollow tentacles, and passes oxygen from the tentacles to the trunk coelom. The body wall is strong and muscular; when threatened, sipunculids can retract their body into a shape resembling a peanut kernel. This is where the name "peanut worm" comes from.
The 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, ribbon worms, and four other phyla.
The fossil record of the Sipuncula is, not surprisingly for soft-bodied animals, sparse. Some scientists, however, consider that hyoliths, 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.