In the family Themistidae the tentacles form an elaborate crown-like structure, the members of this group being specialized filter feeders, unlike the other groups of sipunculans which are deposit feeders. The tentacles are hollow and are extended via hydrostatic pressure in a similar manner as the introvert, but have a different mechanism from that of the rest of the introvert, being connected, via a system of ducts, to one or two contractile sacs next to the oesophagus. Hooks are often present near the mouth on the introvert. These are proteinaceous, non-chitinous specializations of the epidermis, either arranged in rings or scattered. They may be involved in scraping algae off rock, or alternatively provide anchorage. Three genera ("Aspidosiphon", "Lithacrosiphon" and "Cloeosiphon") in the Aspidosiphonidae family possess epidermal structures, known as anal and caudal shields. These are patches of thickened, hard plates, and are used for boring into rock; the anal shield is near the anteriorly-located anus on the trunk, just below the introvert of the animal, while the caudal shield is at the posterior of the body. In "Aspidosiphon" and "Lithacrosiphon" the anal shield is restricted to the dorsal side, causing the introvert to emerge at an angle, whereas it surrounds the anterior trunk in "Cloeosiphon" with the introvert emerging from its center. In "Aspidosiphon" the shield is a hardened, horny structure; in "Lithacrosiphon" it is a calcareous cone; in "Cloeosiphon" it is composed of separate plates. When the introvert is retracted in these animals, the anal shield blocks the entrance to its burrow. At the posterior end of the trunk, a hardened caudal shield is sometimes present in "Aspidosiphon"; this may help with anchoring the animal in its burrow or may be used in the boring process. The digestive tract of sipunculans starts with the esophagus, located between the introvert retractor muscles. In the trunk the intestine runs posteriorly, forms a loop and turns anteriorly again. The downward and upward sections of the gut are coiled around each other, forming a double helix. At the termination of the gut coil, the rectum emerges and ends in the anus, located in the anterior third of the trunk.