Absence of a coelom: Acoelomate animals lack a coelom, which is a fluid-filled body cavity located between the body wall and the digestive tract. Instead, they have a solid body plan with parenchyma, a type of connective tissue that fills the space between the organs.
Bilateral symmetry: Acoelomates exhibit bilateral symmetry, meaning their bodies can be divided longitudinally into two mirror-image halves. This symmetry allows for specialized head and tail regions and facilitates directional movement.
Triploblastic organization: Acoelomate animals are triploblastic, meaning their bodies consist of three embryonic germ layers: the ectoderm, mesoderm, and endoderm. These layers give rise to different tissues and organs during development.
External fertilization and indirect development: Acoelomates typically reproduce sexually through external fertilization, where eggs are released into the environment and fertilized by sperm. Fertilization results in the formation of a zygote, which develops into a free-swimming larva. The larva then undergoes metamorphosis to transform into the adult form.
Simple body plan: Acoelomate animals generally have a simple body plan, lacking complex organ systems and structures. They may possess a rudimentary digestive system, a nervous system consisting of a nerve net or ganglia, and sensory structures such as eyespots or sensory cells.
Examples of acoelomate animals include flatworms (Platyhelminthes) such as planarians and tapeworms, and rotifers (Rotifera). These animals occupy diverse habitats, ranging from freshwater and marine environments to parasitic lifestyles within other organisms.