Spiralia

Spiralia is a large and morphologically diverse clade of protostome animals, uniting phyla including the molluscs, annelid worms, and flatworms (platyhelminths) among many others, alongside a range of smaller, less familiar lineages. The name refers to the diagnostic pattern of spiral cleavage seen in early embryonic development across most member phyla, in which the mitotic spindles of dividing cells are oriented obliquely rather than at right angles, producing a distinctive rotated arrangement of daughter cells. This shared developmental signature, alongside molecular phylogenetic evidence, groups Spiralia within the broader clade Lophotrochozoa. Because Spiralia spans such an enormous range of body plans, from burrowing worms to squid to parasitic flukes, it functions in classification chiefly as a deep evolutionary grouping above the level of any single familiar body plan.