spiralian

Spiralian is a proposed superphylum within the animal kingdom that includes a vast array of invertebrates. Historically, these animals were divided into various phyla, but molecular phylogenetic studies have suggested that many of them share a common ancestry, leading to the concept of Spiralia. The name "Spiralian" is derived from the characteristic spiral cleavage pattern observed in the early embryonic development of many of these organisms. This developmental trait, where the blastomeres divide in a helical manner, is a key piece of evidence supporting their monophyletic status.

The group encompasses a remarkable diversity of forms and lifestyles. Major phyla traditionally placed within Spiralia include Annelida (segmented worms), Mollusca (snails, clams, squid), Nemertea (ribbon worms), Platyhelminthes (flatworms), and Rotifera (rotifers). Other smaller phyla are also considered part of this clade. Spiralians exhibit a wide range of habitats, from marine and freshwater environments to terrestrial ecosystems. Their body plans are varied, with some possessing bilateral symmetry and others exhibiting radial symmetry, though the ancestral state is thought to be bilateral. Their feeding strategies are also diverse, ranging from filter feeders and herbivores to active predators. The ongoing study of spiralian phylogeny continues to refine our understanding of animal evolution and the relationships between these diverse groups.