Protostome [protostomia]

Protostomia (/ˌproʊtəˈstoʊmi.ə/) 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.[1][2] Some examples of protostomes are nematodes, arthropods, flatworms, annelids and molluscs.

Together with the Deuterostomia and Xenacoelomorpha, these form the clade Bilateria, animals with bilateral symmetry and three germ layers.[3]

The most important common feature of this grouping is the homology between the blastopore and the mouth region of the embryo. This means that the blastopore formed by gastrulation generates in the later stages of growth directly the mouth, while in deuterostomes (echinoderms and chordates) corresponds to the anal region.

Metamerism, when present, is almost complete and involves almost all organic systems. The nervous system is typically ganglionic, consisting of supraintestinal cephalic ganglia and subintestinal ganglionic nerve chain. For this reason they are also called gastroneural. Blood circulation typically occurs in the cephalo-caudal sense in the ventral vessels and in the caudo-cephalic sense in the dorsal vessel. There is never a dermaskeleton of mesodermal origin.

Embryonic development occurs through determinative segmentation, primarily spiral. The mesoderm originates from a single initial cell, segregated during egg segmentation. The coelom forms by fissuring within the mesoderm cell mass (schizocelia). Typical larval stages are the trochophore or trochophore-like forms, ciliated “spinning top”.

Phyla typical and most species-rich are the annelids and arthropods. The mollusks, although protostomes, began to diverge relatively early in the schizocelic branch, and this is evidenced by little or no segmentation of their bodies.


  1. Hejnol, A.; Martindale, M. Q. (2009). “The mouth, the anus, and the blastopore – open questions about questionable openings”. In M. J. Telford; D. T. J. Littlewood (eds.). Animal Evolution — Genomes, Fossils, and Trees. pp. 33–40.
  2. Martín-Durán, José M.; Passamaneck, Yale J.; Martindale, Mark Q.; Hejnol, Andreas (2016). “The developmental basis for the recurrent evolution of deuterostomy and protostomy”. Nature Ecology & Evolution. 1 (1): 0005. doi:10.1038/s41559-016-0005. PMID 28812551. S2CID 90795.
  3. Hejnol, A.; Obst, M.; Stamatakis, A.; Ott, M.; Rouse, G. W.; Edgecombe, G. D.; et al. (2009). “Assessing the root of bilaterian animals with scalable phylogenomic methods”. Proceedings of the Royal Society B: Biological Sciences. 276 (1677): 4261–4270. doi:10.1098/rspb.2009.0896. PMC 2817096. PMID 19759036.

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