Spiralians

Spiralians are a hypothetical group of organisms that exhibit a specific developmental pattern known as spiral cleavage. This pattern is characterized by the diagonal orientation of cleavage planes relative to the polar axis of the egg, resulting in daughter cells that are offset from one another. This spiralian development is a key feature observed in many invertebrate phyla, particularly within the protostome lineage.

The arrangement of cells during spiralian cleavage often leads to the formation of a spiral pattern when viewed from the pole of the egg. This is in contrast to the radial cleavage seen in deuterostomes, where cleavage planes are parallel or perpendicular to the polar axis, resulting in cells that are stacked directly on top of each other. The early embryonic development of spiralians typically involves the formation of a blastula followed by gastrulation, leading to the establishment of the three primary germ layers: ectoderm, mesoderm, and endoderm.

Spiralian development is a significant characteristic used in phylogenetic studies to group and understand evolutionary relationships among invertebrates. While the precise evolutionary origins and full extent of the spiralian clade are still areas of ongoing research, its presence unites a diverse array of animal groups, including mollusks, annelids, arthropods, and various smaller phyla. The underlying genetic and molecular mechanisms controlling spiral cleavage are also subjects of scientific investigation, aiming to unravel the developmental pathways that give rise to this distinctive embryonic pattern.