acoelomorphs

Acoelomorphs are a group of small, bilaterally symmetrical marine invertebrates. They are characterized by the absence of a true coelom, a body cavity that is lined by mesoderm in other bilaterian animals. Instead, acoelomorphs possess a parenchymal tissue that fills the space between their organs. They also lack a through-gut, having a blind gut that opens to the exterior through a single pharyngeal opening. Their body plan is relatively simple, with a flattened dorsoventral shape and a distinct head region.

The taxonomic position of acoelomorphs has been a subject of considerable debate. Historically, they were often placed within the phylum Platyhelminthes, the flatworms. However, molecular phylogenetic studies have revealed that acoelomorphs represent a distinct lineage, potentially branching off very early in the evolution of bilaterian animals, possibly even preceding the split between protostomes and deuterostomes. This placement suggests they may be basal bilaterians, offering insights into the ancestral body plan of this major animal group.

Despite their simple morphology, acoelomorphs exhibit a range of ecological roles in marine environments, often inhabiting interstitial spaces in sediments or living as commensals on other organisms. Their reproductive strategies vary, with both sexual and asexual reproduction observed. Further research into their genetics and developmental biology continues to shed light on their evolutionary history and their significance in understanding the diversity of animal life.