Platyzoa

Platyzoa is a proposed group of protostome animals that encompasses Platyhelminthes (flatworms), and a number of phyla previously associated with the Aschelminthes. The exact composition and phylogenetic relationships within the Platyzoa remain a subject of ongoing research and debate, but it typically includes at least the following phyla:

• Platyhelminthes (flatworms)
• Gnathostomulida (jaw worms)
• Rotifera (rotifers)
• Acanthocephala (spiny-headed worms)
• Gastrotricha (hairybacks)

The grouping is largely based on molecular phylogenetic studies. Morphological evidence supporting Platyzoa is less strong, and some analyses have questioned its validity. One proposed synapomorphy (shared derived character) is the lack of a true body cavity (coelom) in the ancestral platyzoan. Many Platyzoan groups are small or microscopic, and some exhibit unusual developmental patterns.

The relationships within Platyzoa are also contested. Some analyses place Platyhelminthes as the basal group, while others suggest a closer relationship between Rotifera and Acanthocephala (the Syndermata). The exact placement of Gastrotricha also varies among different phylogenetic studies. The monophyly of Platyzoa itself has also been questioned in some studies, with some analyses suggesting that Platyhelminthes may be more closely related to Lophotrochozoa.

The significance of the Platyzoa hypothesis lies in its implications for understanding the evolution of body plan and developmental strategies within protostomes. If Platyzoa is a valid clade, it implies that the coelom was either lost in the platyzoan lineage, or that it evolved independently in other protostome groups.