Anatomy of a typical acoel

In a microscope preparation, where an animal is mounted on a microscope slide with a coverslip covering it and squeezing it slightly, acoel turbellarians usually lie ventral-side-down. Within the usually glass-clear body of an acoel, visible parts include a statocyst, which appears as a clear refractile sphere at the anterior end and, in mature animals, parts of the reproductive systems. Details of the reproductive organs, which are important in taxonomy of acoels, are best seen in sectioned material--that is, animals that have been embedded in resin or wax and sliced into serially ordered sections--but the general arrangement and some essential details of these organs can be seen in whole mounts of animals that are gently squeezed by the pressure of a coverslip.

The scheme below shows anatomy as it would be seen in such a ventral-side-down animal (dorsal view). Labels for parts of the animal are revealed by holding the cursor over the blue hyperlinked system-names on the right. For a scheme of a lateral view of an acoel, better showing three-dimensional relationships of some internal organs, click here.
photo of Isodiametra earnhardti scheme of acoel like Isodiametra earnhardti
female reproductive system
male reproductive system
central nervous system, etc.
ventral side
Isodiametra earnhardti
(Hooge & Smith, 2003)
Artwork by Kathryn Apse

The acoels are small ciliated worms without a lumen to the gut. The epidermis is a simple epithelium bearing the cilia and is underlaid by a grid-work of muscles. The cilia provide gliding and swimming movement, and the muscles, both those of the body wall and deeper-lying ones crossing through the body, steer the body with turning and twisting motions as well as pull the body wall in movements that grab food and stuff it through the mouth.

The body wall is richly glandular, with many mucus-producing glands opening between epidermal cells and typically arranged in rows down the body; their rod-like secretions, called rhabdoids, make the cells bearing them look like banana-clusters. The largest of the mucous glands are frontal glands, often voluminous and occupying much of the anterior part of the body and discharging through a pore at the exact anterior pole of the animal. These are sometimes colored, giving the body its color. Many species, especially of tropical environments, bear symbiotic zooxanthellae that give the body a brown color. A few species, too, have pigment in deeper tissues, but most acoels are colorless and glass-clear.

The lack of a lumen in the gut gives the group its name ('Acoela' means without a cavity). Instead of an epithelially lined gut, the digestive tissue consists of a syncytium within which vacuoles form around food that is ingested. The mouth in most acoels lies on the ventral side about mid-body; some have the mouth near the anterior tip of the body, some have it more posteriorly, including one group (the Diopisthoporidae) with a mouth at the extreme posterior tip of the body. The central syncytium has many nuclei because it is formed from cells that fuse with it, continually renewing it, from the cellular parenchyma that surrounds it, a tissue more or less filling the space between the body wall and the central syncytium. Some of the parenchymal cells, particularly in sand-dwelling acoels, are relatively large and fluid-filled---that is, with a large fluid-filled vacuole---and form what is called chordoid tissue, presumably a shock-absorbing tissue.

The reproductive system is hermaphroditic, having both female and male reproductive organs in the same individual animal. The germ cells composing the ovaries and testes lie within the parenchyma, usually near the front end of the body closely apposed to the central syncytium (in the case of the ovaries) or near the dorsal body wall (in the case of the testes), and they undergo meiosis and produce gametes (sperm and eggs) that mature as they move posteriorly through the parenchyma. The male system typically has a well-developed copulatory organ, with muscles and glands that provide a means of delivering the sperm to a partner through the male genital pore, and typically with a seminal vesicle for storing the sperm until ready for copulation. The female system has no oviducts, but there is usually a female pore opening into a vagina that connects with a sperm-storage organ called a bursa. Sperm from the bursa are delivered to the eggs one at a time by a special sperm-sorting structure, the bursal nozzle (bursa mouthpiece). Lacking an oviduct, the animal must lay its eggs through ruptures in the body wall (which are quickly healed) or possibly through the mouth in some species.

The nervous system is largely in the form of a nerve net, but a centralized mass surrounding the statocyst constitutes a brain from which stronger nerves extend as longitudinal cords. These cords link to the nerve nets in the base of the epidermis and beneath the muscles. In some species, the brain is little more than a concentration of the nerve net close to the body wall at the anterior pole of the animal.