I. The longitudinal muscles in the body-wall divided into 17-41 distinct bundles. Four retractor muscles.
A. Body covered with papillae. Numerous filiform tentacles which seldom (or never?) surround the mouth, but stand above and dorsal to it in a horse-shoe, with the opening dorsal. No rectal caecum. Hooks usually present. Four retractors (in Ph. Rupellii only two?). Heart almost always without caeca. Eye-spots always present. Eggs oval, flat, reddish. Almost entirely small tropical species
1. Phymosoma
B. Body devoid of papillae. Tentacular membrane surrounds the mouth in a circlet. Rectum with one or more caeca (except S. edulis?). Hooks absent except in S. australis. Eggs spherical. The tentacular membrane contains a vascular network. A ventral contractile vessel usually present in addition to the heart. Mostly large forms. Found in all seas
2. Sipunculus
II. The longitudinal muscles in the body-wall form a continuous sheath, and are not split up into bundles.
A. Two brown tubes. Numerous tentacles form a wreath round the mouth. Alimentary canal forms a complete spiral, free behind except in Ph. Hanseni. Spindle-muscle usually present. One or more ligaments present, but only on the anterior convolutions of the intestine. Adhesive papillae always absent. Hooks very frequently absent. Eggs spherical. Found in all seas.
3. Phascolosoma
B. Two free brown tubes. Only four or six plumed tentacles. A complete intestinal spiral, not attached behind. Spindle-muscle always present. One or more ligaments present, but only on the anterior convolutions of the intestine. Hooks are present, but sometimes fall off early in life. Heart usually bears caeca. Found only in the tropics.
4. Dendrostoma