- In Aspidosiphon and Physcosoma the tentacles are usually arranged in a horse-shoe, which may be double, overhanging the mouth dorsally.
- (vi.) Aspidosiphon, with 19 species, is easily distinguished by a calcareous deposit and thickened shield at the posterior end and at the base of the introvert, which is eccentric. (vii.) Cloeosiphon has a calcareous ring, made up of lozenge-shaped plates, round the base of its centric introvert.