Identification Photos: Flatworm, Pseudoceros sp.: The Flatworms are members of the Flatworm or Platyhelminth Phylum of which there are approximately 25,000 known species. They range in size from microscopic to several feet long, and they are all very thin. They are the simplest of the worm group, and can be free living or parasitic.

Flatworms are found in both fresh and marine waters. The most well known member is the parasitic tape worm, which lives in humans and is capable of reaching several feet in length.

The marine flatworms are known as polycladids, some of which are very colorful, free living, reaching 15 cm in length. They are invertebrates with soft bodies. The body has a high surface to volume ratio and has no skeletal structure.

The Flatworms have a bilateral symmetry and are triploblastic, composed of three fundamental cell layers: outer ectoderm, middle mesoderm, and inner ectoderm. They have no body cavities (acoelomates) other than the gut, and they lack an anus. The same pharyngeal opening takes in food and expels waste.

The gut of the Flatworms are highly branched to allow intracellular food transport to all parts of the body with no cell being too far from the outside, making a flat shape a necessity. They take in oxygen but have no formal respiration system. The nervous system is very simple, being composed of two nerve cords running down each side of the body.

The Flatworms have two simple brains called ganglia, which are bundles of nerves. They do not have formal eyes but have two eyespots, which help them sense light. They move via tiny bristles called cilia and two layers of muscles under the skin. In an emergency they are capable of swimming via rhythmic muscular contractions.

The marine Flatworms are hermaphroditic with each individual producing both eggs and sperm. They can also regenerate missing body parts with ease. They prey upon small worms, insects, and microscopic matter. The marine Flatworms have a small demand from the aquarium trade and as lab animals, but otherwise they are not of significance to humans.

Flatworm, Pseudoceros sp.: Collections made under a large rock at low tide at Km. 17, El Tule, San Jose del Cabo, Baja California Sur, Mexico, April 2007. It looked like floating lettuce, but it was “swimming”! A unique collection made at midmorning. Size approximately 1 inch. Tentative identification courtesy of Greg Rouse, Scripps Institute of Oceanography, La Jolla, Calif. Description and photos courtesy of John Snow.