General Sipunculan Anatomy -- The body is completely unsegmented, and the intestine forms a twisted loop, with the anus on the side of the body. Typical of sipunculans is a forward (anterior) body section, the introvert, which can be retracted into the body by the retractor muscles. At the tip of the introvert (retracted in the specimen shown at the top of this page) is the mouth, which is surrounded by a ring of tentacles. The body cavity, or coelom, of sipunculans is large and filled with fluid, in which are found free-floating cells known as hemerythrocytes as well as free-floating clusters of cells known as ciliary urns. Sipunculans have no circulatory or respiratory systems; the coelomic fluid transports both nutrients and oxygen to all parts of the body. Nephridia filter the coelomic fluid. |
Sipunculan Phylogeny and Systematics -- Sipunculans are sometimes thought to be a group of annelids, and have been classified within them. However, this phylogenetic hypothesis is contradicted by comparing the anatomy of annelids, sipunculans, and other animals. For instance, sipunculans have no trace of important annelid characters such as segmentation and chaetae (bristles). The characters they do share with annelids (e.g. worm shape, introvert, trochophore larva) are not restricted to annelids and sipunculans, but are much more general. |
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