C. Only one brown tube, that of the right side, present; it is attached to the body-wall throughout its entire length. Numerous tentacles form a circle round the mouth. The alimentary canal forms no spiral, or an incomplete one. No spindle-muscle, but the intestine is attached to the body-wall throughout its length by numerous ligaments. Adhesive papillae often present. Not more than two retractors. Spherical eggs. Inhabits Mollusc shells or tubes. Found in all seas
5. Phascolion
III. At both ends of the trunk a distinct horny shield, or tube-like cornification, or a calcareous ring at the anterior end of the trunk. Hooks sometimes present. Longitudinal muscles continuous or split up into bundles.
A. A shield at both ends of the trunk. Introvert excentric, arising from the ventral side of the anterior shield. Tentacles small and few in number, arranged in a horse-shoe above the mouth. A spindle-muscle, which arises from the posterior end of the body, traverses the intestinal coil. Two retractors only, these are the ventral; they are frequently fused together from their point of origin.
6. Aspidosiphon
B. A calcareous ring surrounds the anterior end of the trunk, from the middle of which the introvert is extruded. Longitudinal muscles continuous. Hooks bifid. Tropical.
7. Cloeosiphon
C. A corneous ring, from which the introvert issues, surrounds the anterior end of the trunk, and the posterior end of the trunk is produced into a corneous spike. Six pinnate tentacles encircle the mouth. Four retractors. Hooks present on the introvert. Longitudinal muscles continuous. Intestine not coiled throughout in a spiral nor fastened posteriorly. Spindle muscle present.
8. Golfingia
IV. No tentacles, but two leaf-like extensions of the body-wall guard the mouth. Four retractors. Few intestinal loops, quite free. No vascular system.