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
Deuterostomia (; in Greek) are animals typically characterized by their anus forming before their mouth during embryonic development. The group's sister clade is Protostomia, animals whose digestive tract development is more varied. Some ex ...
.
Well known examples of protostomes are
arthropod
Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chiti ...
s,
molluscs,
annelids,
flatworms and
nematodes. They are also called schizocoelomates since
schizocoely
Schizocoely (adjective forms: schizocoelous or schizocoelic) is a process by which some animal embryos develop. The schizocoely mechanism occurs when secondary body cavities ( coeloms) are formed by splitting a solid mass of mesodermal embryonic ...
typically occurs in them.
Together with the Deuterostomia and
Xenacoelomorpha
Xenacoelomorpha is a small phylum of bilaterian invertebrate animals, consisting of two sister groups: xenoturbellids and acoelomorphs. This new phylum was named in February 2011 and suggested based on morphological synapomorphies (physical ...
, these form the clade
Bilateria, animals with
bilateral symmetry, anteroposterior axis and
three
3 is a number, numeral, and glyph.
3, three, or III may also refer to:
* AD 3, the third year of the AD era
* 3 BC, the third year before the AD era
* March, the third month
Books
* '' Three of Them'' (Russian: ', literally, "three"), a 1901 ...
germ layers.
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
The primary gut that forms during gastrulation in a developing embryo is known as the archenteron, the gastrocoel or the primitive digestive tube. It develops into the endoderm and mesoderm of an animal.
Formation in sea urchins
As primary mesen ...
). 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 'first' + '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. Many protostomes (the
Spiralia clade) undergo spiral
cleavage during cell division instead of radial cleavage.
Spiral cleavage happens because the cells' division planes are angled to the polar major axis, instead of being parallel or perpendicular to it.
Another difference is that secondary body cavities (
coeloms) generally form by
schizocoely
Schizocoely (adjective forms: schizocoelous or schizocoelic) is a process by which some animal embryos develop. The schizocoely mechanism occurs when secondary body cavities ( coeloms) are formed by splitting a solid mass of mesodermal embryonic ...
, where the coelom forms out of a solid mass of embryonic tissue splitting away from the rest, instead of by
enterocoelic pouching, 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. The two clades diverged about 600 million years ago. Protostomes evolved into over a million species alive today, compared to about 60,000 deuterostome species.
Protostomes are divided into the
Ecdysozoa (e.g.
arthropods,
nematodes
The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-parasitic nematodes also known as eelworms. They are a diverse animal phylum inhabiting a broa ...
) and the
Spiralia (e.g.
molluscs
Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is estim ...
,
annelids
The annelids (Annelida , from Latin ', "little ring"), also known as the segmented worms, are a large phylum, with over 22,000 extant species including ragworms, earthworms, and leeches. The species exist in and have adapted to various ecolog ...
,
platyhelminths
The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") are a phylum of relatively simple bilaterian, unsegmen ...
, and
rotifers
The rotifers (, from the Latin , "wheel", and , "bearing"), commonly called wheel animals or wheel animalcules, make up a phylum (Rotifera ) of microscopic and near-microscopic pseudocoelomate animals.
They were first described by Rev. John H ...
). A modern consensus
phylogenetic tree for the protostomes is shown below.
The timing of clades radiating into newer clades is given in
mya (millions of years ago); less certain placements are indicated with dashed lines.
See also
*
Embryological origins of the mouth and anus
The embryological origin of the mouth and anus is an important characteristic, and forms the morphological basis for separating bilaterian animals into two natural groupings: the protostomes and deuterostomes.
In animals at least as complex as ...
The Taxonomiconfor
Karl Grobben
References
External links
*
{{Authority control
Ediacaran first appearances
Infrakingdoms
Taxa named by Karl Grobben