The Spiralia are a morphologically diverse

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English ter ...

protostome 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 me ...

animals, including within their number the molluscs, annelids, platyhelminths and other taxa. The term ''Spiralia'' is applied to those phyla that exhibit canonical

spiral cleavage In embryology, cleavage is the division of cells in the early development of the embryo, following fertilization. The zygotes of many species undergo rapid cell cycles with no significant overall growth, producing a cluster of cells the same size ...

, a pattern of early development found in most (but not all) members of the

Lophotrochozoa Lophotrochozoa (, "crest/wheel animals") is a clade of protostome animals within the Spiralia. The taxon was established as a monophyletic group based on molecular evidence. The clade includes animals like annelids, molluscs, bryozoans, bra ...

Distribution of spiralian development across phylogeny

Members of the molluscs, annelids, platyhelminths and

nemertean Nemertea is a phylum of animals also known as ribbon worms or proboscis worms, consisting of 1300 known species. Most ribbon worms are very slim, usually only a few millimeters wide, although a few have relatively short but wide bodies. Many h ...

s have all been shown to exhibit spiral cleavage in its classical form. Other spiralian phyla ( rotifers, brachiopods, phoronids, gastrotrichs, and bryozoans) are also said to display a derived form of

in at least a portion of their constituent species, although evidence for this is sparse.

Lophotrochozoa within Spiralia

Previously,

was thought to be unique to the Spiralia in the strictest sense—animals such as molluscs and annelids which exhibit classical spiral cleavage. The presence of spiral cleavage in animals such as platyhelminths could be difficult to correlate with some phylogenies. Evidence of a close relationship between molluscs, annelids and lophophorates was found in 1995 and Lophotrochozoa was defined as the group containing these taxa and all the descendants of their last common ancestor. More recent research has established the

as a superphylum within the Metazoa. With this understanding, the presence of spiral cleavage in polyclad platyhelminths, as well as the more traditional Spiralia, has led to the hypothesis that spiral cleavage was present ancestrally across the Lophotrochozoa as a whole. With the introduction of Platytrochozoa and Rouphozoa, the cladogram is as follows, with an indication approximately how many million years ago (Mya) the clades radiated into newer clades. An alternative phylogeny was given in 2019, with a basal grouping Mollusca with Entoprocta grouping named Tetraneuralia, and a second grouping of Nemertea with Platyhelminthes named Parenchymia as sister of Annelida. In their proposal and according to the original definition, Lophotrochozoa may become a senior synonym for Platytrochozoa. In 2019 the Rouphozoa was recovered again as a basal Platytrochozoa clade.

References

{{Taxonbar, from=Q1211307 Protostome unranked clades