Protostomia

From Palaeos.org

Choanomonada
`--Metazoa
   `==Porifera (paraphyletic?)
      `==Radiata (paraphyletic?)
         `--Bilateria
            `==Acoelomorpha (paraphyletic?)
               |--Protostomia
               |  |--Ecdysozoa
               |  `--Lophotrochozoa
               `--Deuterostomia

Enallagama cyathigerum, a damselfly (Ecdysozoa, Arthropoda, Insecta, Odonata, Coenagrionidae)

The Protostomes (Greek: protos first, stoma mouth) are so called because in their very early embryonic develeopment, the mouth develops first. This groups includes the great majority of invertebrates, such as arthropods, molluscs, annelids, and many less well-known taxa.

Embryology

The protostome condition is defined by

• a spiral and determinant cleavage in the early stages of embryonic development, which means that the fate of the cells is determined as they are formed.
• schizocoelous coelom formation. As the archenteron (embryonic gut) forms, the coelom begins as splits within the solid mesodermal mass, with the formation of the blastopore (the original opening) into the mouth

In both these attributes they differ from deuterostomes.

Phylogeny

There is a tendency among researchers in the field of molecular phylogeny to divide the Protostomia into two further groups, the Ecdysozoa ("moulting animals") and Lophotrochozoa. However, not all molecular phylogeneticists accept this approach.

To these have more recently been added the Platyzoa as a basal assemblage that may or may not be a clade. The Platyzoa and the Lophotrochozoa. together make up the Spiralia, where the embryo undergoes spiral cleavage.

Credits

MAK020407 Palaeos com; MAK061012 Palaeos org