Protostome
Bilateria
<tr><td>Superphylum:<td>
Protostomia
</table>
Mollusca
Arthropoda
Annelida
</table>
Protostomes (from the
Greek:
first the mouth) are a
superphylum of animals in the taxonomic group
bilateria, and include animals such as
arthropods,
mollusks, and
nematodes. They are most often compared with
deuterostomes, the other major group of bilateria. The major distinctions between deuterostomes and protostomes are found in embryonic development.
In both protostomes and deuterostomes, the embryo consists of a little ball of cells known as a
blastula. Protostomes have their early cell divisions diagonal to the polar axis forming a spiral arrangement of cells; this is called spiral cleavage. A groups of cells move inward to form an opening called the blastophore, which in protostomes develops into the mouth.
Protostomes have a schizocoely, where the
mesoderm splits and the split widens into a cavity that becomes the
coelom.
Current molecular data suggest that protostome animals can be divided into two major groups:
lophotrochozoa and
ecdysozoa.
Phyla in protostomes:
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