Acoels

Acoels are a group of small, marine invertebrates that were once classified within the phylum Platyhelminthes (flatworms). However, modern phylogenetic studies have placed them in their own phylum, Acoelomorpha, due to significant genetic and morphological differences. They are bilaterally symmetrical, unsegmented, and lack a coelom, meaning they have no internal body cavity.

These tiny creatures are typically less than a millimeter in length and inhabit marine sediments, often found in intertidal zones. They possess a simple body plan, with a well-developed epidermis and gastrodermis, but lack true organ systems such as a circulatory or excretory system. Instead, nutrients are distributed through a simple, branched gut, and waste is expelled through the mouth.

Acoels have a single opening that serves as both mouth and anus. They possess a statocyst for balance and two ocelli (eyespots) for light detection, indicating a degree of sensory capability. Reproduction can be both sexual and asexual, with some species exhibiting hermaphroditism.

Despite their seemingly simple structure, acoels exhibit a surprising diversity in their morphology and ecological roles. They are important components of benthic ecosystems, contributing to nutrient cycling. Their unique evolutionary position makes them of significant interest to researchers studying the origins and evolution of animal body plans.