| Sipuncula |
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| This
phylum's 'place' relative to other phyla:
|
Selected
taxa within this taxon:
none (4 classes) |
Sipunculids are burrowing worms that share many similarities with annelids: possess a trochophore larvae and similar musculature, excretory, and nervous systems (and similar ecologically with echiurans). Also closely related based on molecular data.
But sipunculids lack segementation and
setae
Defining characteristics
Why are sipunculids not considered
annelids?
More broadly, is the grouping of animals into phyla purely arbitrary
(is
the concept of 'body plan' subjective). Should classification
schemes
be designed for convience as well as reflect evolutionary relationships?
While evolution does not produce an unbroken
continuum of organisms, some "breaks" (branches) are wider (further
apart)
than others.
In addition, some zoologists have argued that the sipunculans more closely related to the molluscs than to annelids, based on similarity early in development (arrangement of cells in the embryo, known as the "molluscan cross" and in sipunculid larvae a muscular creeping foot)
Fossil of Sipuncula are typiclly
rare
with one possible exception. Hyoliths (Paleozoic)
are
conical shells with hinged lids. In a few species the intestine
has
been fossilized and is looped and coiled as in living sipunculans. A
few
living sipunculans secrete a calcified cuticular plate, the anal shield.
Lecture Sources:
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