Protostomia () is the clade of animals once thought to be characterized by the formation of the organism's mouth before its anus during embryonic 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.
Together with the Deuterostomia and Xenacoelomorpha, these form the clade Bilateria, animals with bilateral symmetry, anteroposterior axis and triploblastic .
Protostomy
In animals at least as complex as
earthworms, the first phase in gut development involves the
embryo forming a dent on one side (the
blastopore) which deepens to become its digestive tube (the
archenteron). In the sister-clade, the deuterostomes (), the original dent becomes the
anus while the gut eventually tunnels through to make another opening, which forms the
mouth. The protostomes (from Greek πρωτο- 'first' + στόμα '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.
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 name
protostome is disputable.
Protostome and deuterostome embryos differ in several other ways. Secondary body cavities () generally form by schizocoely, where the coelom forms out of a solid mass of embryonic tissue splitting away from the rest, instead of by Enterocoely, where the coelom would otherwise form out of in-folded gut walls.
Evolution
The common ancestor of protostomes and deuterostomes was evidently a worm-like aquatic animal of the
Ediacaran biota. The two clades diverged over 600 million years ago. Protostomes evolved into over a million species alive today, compared to ca. 73,000 deuterostome species.
[The Invertebrate tree of life, Giribet & Edgecombe, 2020; p.155]
Protostomes are divided into the Ecdysozoa (e.g. arthropoda, nematoda) and the Spiralia (e.g. mollusca, annelida, platyhelminthes, and rotifera). A modern consensus phylogenetic tree for the protostomes is shown below.
See also
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Embryological origins of the mouth and anus
-
Urbilaterian, a hypothethical common ancestor to Protostomes and Deuterostomes
External links
