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Protostome
 
 
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Protostomia (from the Greek
Greek language

Greek is an Indo-European languages native to the southern Balkan peninsula, the language of the Greek people. It forms an independent branch within Indo-European....
: mouth first) are a clade
Clade

A clade is a term used in modern alpha taxonomy, the scientific classification of living and fossil organisms, to describe a monophyletic group, defined as a group consisting of a single common ancestor and all its descendants.The term "monophyletic group" is used in this article in the conventional sense of "an a...
 of animal
Animal

Animals are a major group of multicellular, eukaryotic organisms of the Kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life....
s. Together with the deuterostome
Deuterostome

Deuterostomes are a superphylum of animals. They are a taxon of the Bilateria branch of the subregnum Eumetazoa, and are opposed to the protostomes....
s and a few smaller phyla
Phylum

A phylum "Phylum" is adopted from the Greek phylai, the clan-based voting groups in Greek city-states. is a taxonomic rank below Kingdom and above Class ....
, they make up the Bilateria
Bilateria

The Bilateria are all animals having a symmetry #Bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside....
, mostly comprising animals with bilateral symmetry
Symmetry (biology)

Symmetry in biology is the balanced distribution of duplicate body parts or shapes. The body plans of most multicellular organisms exhibit some form of symmetry, either radial symmetry or bilateral symmetry or glide symmetry....
 and three germ layer
Germ layer

A germ layer is a group of cell s, formed during animal embryogenesis. Germ layers are particularly pronounced in the vertebrates; however, all animals more complex than sea sponge produce two or three primary tissue layers ....
s. The major distinctions between deuterostome
Deuterostome

Deuterostomes are a superphylum of animals. They are a taxon of the Bilateria branch of the subregnum Eumetazoa, and are opposed to the protostomes....
s and protostomes are found in embryonic development.

In animals at least as complex as an earthworms, the embryo
Embryo

An embryo is a multicellular organism ploidy eukaryote in its earliest stage of development, from the time of first cell division until birth, Egg , or germination....
 forms a dent on one side, the blastopore
Blastopore

A blastopore is an opening into the archenteron during the embryonic stages of an organism. The Embryological origins of the mouth and anus protostomes and deuterostomes is based on the direction in which the mouth develops in relation to the blastopore....
, which deepens to become the archenteron
Archenteron

The primitive gut that forms during gastrulation in the developing blastula is known as the archenteron. It develops into the digestive tract of an animal....
, the first phase in the growth of the gut.






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Protostomia (from the Greek
Greek language

Greek is an Indo-European languages native to the southern Balkan peninsula, the language of the Greek people. It forms an independent branch within Indo-European....
: mouth first) are a clade
Clade

A clade is a term used in modern alpha taxonomy, the scientific classification of living and fossil organisms, to describe a monophyletic group, defined as a group consisting of a single common ancestor and all its descendants.The term "monophyletic group" is used in this article in the conventional sense of "an a...
 of animal
Animal

Animals are a major group of multicellular, eukaryotic organisms of the Kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life....
s. Together with the deuterostome
Deuterostome

Deuterostomes are a superphylum of animals. They are a taxon of the Bilateria branch of the subregnum Eumetazoa, and are opposed to the protostomes....
s and a few smaller phyla
Phylum

A phylum "Phylum" is adopted from the Greek phylai, the clan-based voting groups in Greek city-states. is a taxonomic rank below Kingdom and above Class ....
, they make up the Bilateria
Bilateria

The Bilateria are all animals having a symmetry #Bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside....
, mostly comprising animals with bilateral symmetry
Symmetry (biology)

Symmetry in biology is the balanced distribution of duplicate body parts or shapes. The body plans of most multicellular organisms exhibit some form of symmetry, either radial symmetry or bilateral symmetry or glide symmetry....
 and three germ layer
Germ layer

A germ layer is a group of cell s, formed during animal embryogenesis. Germ layers are particularly pronounced in the vertebrates; however, all animals more complex than sea sponge produce two or three primary tissue layers ....
s. The major distinctions between deuterostome
Deuterostome

Deuterostomes are a superphylum of animals. They are a taxon of the Bilateria branch of the subregnum Eumetazoa, and are opposed to the protostomes....
s and protostomes are found in embryonic development.

In animals at least as complex as an earthworms, the embryo
Embryo

An embryo is a multicellular organism ploidy eukaryote in its earliest stage of development, from the time of first cell division until birth, Egg , or germination....
 forms a dent on one side, the blastopore
Blastopore

A blastopore is an opening into the archenteron during the embryonic stages of an organism. The Embryological origins of the mouth and anus protostomes and deuterostomes is based on the direction in which the mouth develops in relation to the blastopore....
, which deepens to become the archenteron
Archenteron

The primitive gut that forms during gastrulation in the developing blastula is known as the archenteron. It develops into the digestive tract of an animal....
, the first phase in the growth of the gut. In deuterostome
Deuterostome

Deuterostomes are a superphylum of animals. They are a taxon of the Bilateria branch of the subregnum Eumetazoa, and are opposed to the protostomes....
s, the original dent becomes the anus while the gut eventually tunnels through to make another opening, which forms the mouth. The protostome
Protostome

Protostomia are a clade of animals. Together with the deuterostomes and a few smaller phylum, they make up the Bilateria, mostly comprising animals with symmetry #Bilateral symmetry and triploblastic germ layers....
s were so named because it used to be thought that in their embryos the dent formed the mouth while the anus was formed later, at the opening made by the other end of the gut. More recent research, however, shows that in protostomes the edges of the dent close up in the middle, leaving openings at the ends which become the mouth and anus. However, this idea has been challenged, because the platyhelminthes, a group which forms a sister group to the rest of the bilaterian animals, have a single mouth which leads into a blind gut (with no anus). The genes employed in the embryonic construction of this mouth are the same as those expressed around the protostome mouth,

There are other significant differences between the protostome and deuterostome patterns of development:
  • Most protostomes are schizocoelomates, meaning a solid mass of the embryonic mesoderm
    Mesoderm

    One of the three germ layers found in the embryos of animals more complex than cnidarians, making them triploblastic. Mesoderm forms in the embryo during gastrulation when some of the cells migrating inward to form the endoderm, produce an additional layer that lies between the endoderm and the ectoderm....
     splits to form a coelom
    Coelom

    The coelom is a fluid filled cavity formed within the mesoderm. Coeloms developed in triploblasts but were subsequently lost in several lineages....
    . A few, such as Priapulids, have no coelom, but they may have descended from schizocoelomate ancestors. On the other hand all known deuterostomes are enterocoelous
    Enterocoely

    Enterocoely is a process by which a mesoderm is formed in a developing embryo, in which the coelom forms from pouches "pinched" off of the digestive tract ....
    , meaning that the coelom is formed from longitudinal pouches of the archenteron
    Archenteron

    The primitive gut that forms during gastrulation in the developing blastula is known as the archenteron. It develops into the digestive tract of an animal....
     which then become separate cavities.
  • Within the Protostomes a number of phyla undergo what is known as spiral cleavage which is determinate, meaning that the fate of the cells is determined as they are formed. This is in contrast to deuterostomes which have radial cleavage that is indeterminate.


Current molecular data suggest that protostome animals can be divided into three major groups:
  • Ecdysozoa
    Ecdysozoa

    The Ecdysozoa are a grouping of protostome animals, including the Arthropoda , roundworm, and several smaller phylum . They were first defined by Aguinaldo et al. in 1997, based mainly on trees constructed using 18S ribosomal RNA genes....
    , e.g. arthropod
    Arthropod

    Arthropods are animals belonging to the Scientific classification Arthropoda , and include the insects, arachnids, crustaceans, and others....
    s, nematode
    Nematode

    The "roundworms" or "nematodes" are the most diverse phylum of body cavity, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 80,000 have been described, of which over 15,000 are parasite....
    s
  • Platyzoa
    Platyzoa

    The Platyzoa are a group of protostome animals proposed by Thomas Cavalier-Smith in 1998. Cavalier-Smith included in Platyzoa the Phylum flatworm or flatworms, and a new phylum, Acanthognatha, into which he gathered several previously described phyla of microscopic animals....
    , e.g. platyhelminthes, rotifer
    Rotifer

    The rotifers make up a phylum of microscopic and near-microscopic body cavity animals. They were first described by Rev. John Harris in 1696 and other forms were described by Anton van Leeuwenhoek in 1703....
    s
  • Lophotrochozoa
    Lophotrochozoa

    The Lophotrochozoa are one of three major groupings of protostome animals. The taxon was introduced in 1995 in a paper by Kenneth M Halanych et al based on molecular data....
    , e.g. molluscs, annelid
    Annelid

    The annelids, collectively called Annelida , are a large Scientific classification of animals comprising the segmented worms, with about 15,000 modern species including the well-known earthworms and leeches....
    s
as well as a number of minor taxa of basal or ambiguous affinity.

See also

  • Deuterostome
    Deuterostome

    Deuterostomes are a superphylum of animals. They are a taxon of the Bilateria branch of the subregnum Eumetazoa, and are opposed to the protostomes....
  • Embryological origins of the mouth and anus
    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 clades: the protostomes and deuterostomes....
  • for Karl Grobben
    Karl Grobben

    Karl Grobben was an Austrian biologist, who was born on August 27 1854 in Brno, and died on April 13 1945 in Salzburg. He graduated from, and later worked at, the University of Vienna, chiefly on molluscs and crustaceans....