Lahille, 1888
Trunk short, with its fore part depressed (Kowalevskiidae-trunk). Endostyle lacking; 4 rows of one-celled ciliated processes instead. Gill slits elongate, ellipsoidal. Esophagus entering the stomach frontally. Stomach formed by a few large cells. Heart lacking. Tail spindle-shaped (Kowalevskiidae).