Physical characteristics

Acoels are tiny; the members of most species are no longer than 0.078 in (2 mm), although Convolutriloba retrogemma can reach lengths of 0.23-0.28 in (6-7 mm) and Convoluta roscoffensis can grow up to 0.59 in (15 mm) long. The bodies

Acoels (Waminoa sp.) on hard coral, being sucked up by the sea slug (Chelidonura electra), Coral Bay, WA. (Photo by A. Flowers & L. Newman. Reproduced by permission.)

of acoels may be either oval or cylindrical in shape, and flattened dorsoventrally.

Acoels have either a simple pharynx or none at all; the pharynx or mouth is situated on the ventral (lower) surface. They have no digestive tract (gut), no protonephridia (primitive kidneys for excretion and osmotic balance), and no distinct gonads. The brain is quite simple, unlike the more complex bilobed brain found in most platyhelminth species. The acoel nervous system is a loose net of nerve fibers strung throughout the body. Most species also have simple eyes known as ocelli. Individuals of nearly all species carry a sta-tocyst, a tiny, spherical organ for balance and orientation.

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