Protostomia (/ˌproʊtəˈstoʊmi.ə/) is theclade of animals once thought to be characterized by the formation of the organism's mouth before its anus duringembryonic development. This nature has since been discovered to be extremely variable among Protostomia's members, although the reverse is typically true of its sister clade,Deuterostomia.[1][2] Well-known examples ofprotostomes arearthropods,molluscs,annelids,flatworms andnematodes. They are also calledschizocoelomates sinceschizocoely typically occurs in them.
In animals at least as complex asearthworms, the first phase ingut development involves theembryo forming a dent on one side (theblastopore) which deepens to become its digestive tube (thearchenteron). In the sister-clade, the deuterostomes (lit.'second-mouth'), the original dent becomes theanus while the gut eventually tunnels through to make another opening, which forms themouth. The protostomes (from Greekπρωτο-prōto- 'first' +στόμαstóma 'mouth') were so named because it was once believed that in all cases the embryological dent formed the mouth while the anus was formed later, at the opening made by the other end of the gut.[4][1]It is now known that the fate of the blastopore among protostomes is extremely variable; while the evolutionary distinction between deuterostomes and protostomes remains valid, the descriptive accuracy of the nameprotostome is disputable.[1]
Protostome and deuterostome embryos differ in several other ways. Secondary body cavities (coeloms) generally form byschizocoely, where the coelom forms out of a solid mass of embryonic tissue splitting away from the rest, instead of byenterocoelic pouching, where the coelom would otherwise form out of in-folded gut walls.[5]
The common ancestor of protostomes and deuterostomes was evidently a worm-like aquatic animal of theEdiacaran. The two clades diverged about 600 million years ago. Protostomes evolved into over a million species alive today, compared to ca. 73,000 deuterostome species.[6]
^Peters, Kenneth E.; Walters, Clifford C.; Moldowan, J. Michael (2005).The Biomarker Guide: Biomarkers and isotopes in petroleum systems and Earth history. Vol. 2. Cambridge University Press. p. 717.ISBN978-0-521-83762-0.
^Safra, Jacob E. (2003).The New Encyclopædia Britannica, Volume 1; Volume 3. Encyclopædia Britannica. p. 767.ISBN978-0-85229-961-6.
^The Invertebrate tree of life, Giribet & Edgecombe, 2020; p.155
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