The family Molgulidœ was established by Lacaze-Duthiers in 1877, and distinguished from the Cynthiadœ, with which it had formerly been united by Heller.

Lacaze-Duthiers divides the species known to him into four genera,—Anurella, Molgula, Ctenicella, and Eugyra,—of which Anurella forms a sub-family or section the Molgulidœ anurœ, the other three constituting the Molgulidœ urodelœ. The character by which Anurella is distinguished from the other genera is the tail-less amoeboid larva; but this most interesting developmental difference does not seem to be accompanied by any structural features in the adult animal, and consequently it becomes impossible to distinguish the genus unless one examines the animal alive and at the breeding season.