Achelia echinata has a small, flat body up to 0.15-0.2 cm in length bearing a head, a pair of palps, a pair of ovigers and four pairs of legs. The body is oval in shape and divided into four segments that, although ill-defined, are marked on the dorsal surface by a suture between each of the first three segments. The surface of the body is covered with spines that are divided into two branches each bearing smaller branches. The body is coloured yellowish-brown to cream. The head is fused with the first body segment and bears an ocular tubercule, a proboscis, a pair of chelifores and a mouth that is situated within three pointed lips. The ocular tubercule is slightly conical in shape, has a sharp spine at the tip and bears two pairs of eyes, one pair facing forward the other facing backwards. The proboscis is about the same length as the body and is tapered towards the tip. The chelifores are about half the length of the body and bear small, squat chelae. The palps protrude from the first segment and are divided into eight segments. The ovigers extend from the ventral surface of the first segment and are divided into nine segments. The legs extend from lateral processes of each body segment and are approximately three times the length of the body. Each leg terminates with a claw and is divided into eight segments each bearing a number of spines that characteristically consist of one pair on on the lateral process and two pairs on the first segment.Achelia echinata are a slow-moving species. They are carnivorous, suctorial grazers preying on bryozoans, sea anemones, hydroids such as Dynaema pumila and algae such as Griffithsia and Enteromorpha. This species has been known to feed on Flustra foliacea by lying in wait until the bryozoan opens its operculum then putting its proboscis inside the zooid. Reproduction is via external fertilization and the release of protonymphan larvae.
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