Anatomy
Sipunculans are worm-like animals ranging from 2 to 720 millimetres (0.079 to 28 in) in length, with most species being under 10 centimetres (3.9 in). The sipunculan body is divided into an unsegmented trunk and a narrower, retractable anterior section, called the "introvert". Sipunculans have a body wall somewhat similar to that of annelids (though unsegmented) in that it consists of a non-ciliated epidermis overlain by a cuticle, an outer layer of circular and an inner layer of longitudinal musculature. The body wall surrounds the coelom that is filled with fluid on which the body wall musculature acts as a hydrostatic skeleton to extend or contract the animal. When threatened, Sipunculids can retract their body into a shape resembling a peanut kernel - a practice that has given rise to the name "peanut worm". The introvert is retractable into the trunk via two pairs of retractor muscles that extend as narrow ribbons from the trunk wall to attachment points in the introvert. The introvert can be protruded from the trunk by contracting the muscles of the trunk wall, thus forcing the fluid in the body cavity forwards.
The sipunculan mouth is located at the anterior end of the introvert, which is surrounded by a mass of 18 - 24 ciliated tentacles in the Sipunculidea. In the Phascolosomatidea, the tentacles are arranged in an arc around the nuchal organ, also located at the tip of the introvert. The tentacles are used to gather organic detritus from the water or substrate, and probably also function as gills. The tentacles at the tip of the introvert are hollow and are extended via hydrostatic pressure in a similar manner as the introvert, but have a separate system from that of the rest of the introvert; they are 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 which are either arranged in rings or scattered.
Three genera (Aspidosiphon, Lithacrosiphon and Cloeosiphon) possess epidermal modifications, called the anal shield near the anteriorly located anus on the trunk just below the introvert of the animal. 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. At the posterior end, a hardened caudal shield is sometimes present in Aspidosiphon.
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