Acoelomorpha
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Acoelomorpha is a subphylum of very simple and small soft-bodied animals with planula-like features that live in marine or brackish waters. They usually live between grains of sediment, swimming as pl
Waminoa on Plerogyra
Xenoturbella, the sister group to acoelomorphs
Symsagittifera roscoffensis
Bilateria
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Bilateria is a large clade of animals characterised by bilateral symmetry during embryonic development. This means their body plans are laid around a longitudinal axis with a front and a rear end, as
Acoelomorpha
…based on phenotypic features, acoelomorphs were considered to belong to the phylum Platyhelminthes, which was long seen as the sister group to all other bilaterian phyla. However, a series of molecular phylogenetics studies at the hinge between the 20th and 21st centuries demonstrated that they are fast-evolving…
EB1911 Crustacea Fig. 12.—Nauplius of a Prawn
Ikaria wariootia, living 571–539 million years ago, is one of the oldest bilaterians identified.