Synallactes challengeri can grow up to 20 cm in length. Its elongate, cylindrical body tapers toward the posterior end. The dorsal side of the body is covered with long, slender, tapering papillae. They do not appear to be in rows. The animal is grey with a tinge of pink or purple. On the ventral side, there are three series of tube feet. The middle series has two to four rows; the lateral series only has two. Twenty yellowish peltate tentacles surround the subterminal mouth.
Skin ossicles: tables are a cross with a tall spire rising from the centre; rods (not shown) are long and slightly curved with racquet-shaped ends.
Ostergren (1896) placed this species in Synallactes. Clark (1922) also suggested that this species was not a Stichopus, but that it possessed the characters of the family Synallactidae.
Similar SpeciesSynallactes challengeri might be mistaken for a juvenile of a Parastichopus. The body colour, and size and shape or the papillae are fairly distinctive. Internally, S. challengeri has no tentacle ampullae, and the ossicles are quite different.