Diesing, 1851

Description:
A medium- to large-sized species, up to 80 mm in length with dark chitinous shields at both ends of the cylindrical trunk. With age, the specimens becomes darker in colour and large specimens are frequently dark brown or even black. The oral disk has a swollen rim (cephalic collar) and carries 10-12 tentacles, arranged in a crescent and enclosing the nuchal organ. The tentacles are short and united for much of their length. The introvert arises from the ventral margin of the anal shield and is densely covered with small hooks; anteriorly the hooks are thin, triangular in outline and arranged in numerous rings becoming more conical, spine-like and arranged irregularly over the posterior half or two-thirds of the introvert. In Atlantic specimens all hooks have one point but the Mediterranean form is characterized by the anterior-most hooks having two points. The anal (anterior) and caudal (posterior) shields of the trunk are heavily chitinized areas composed of closely packed, rounded or polygonal bodies. The anal shield is radially furrowed over its dorsal half, with raised, wart-like papillae ventrally. The caudal shield has radial furrows and a peripheral zone of prominent papillae. Nephridiopores are ventrolateral and at about the level of the anus on the anterior part of the trunk. The longitudinal muscle layer of the body wall is continuous except in the anal shield region where it forms tendinous bands.
Internally, one retractor muscle is present, inserted by two roots on the ventral margin of the caudal shield (A. muelleri-internal). The intestine is tightly coiled in a double spiral supported by a spindle muscle attached anteriorly near the anus and posteriorly on the dorsal part of the caudal shield. A long fixing muscle fastens the anterior coils of the gut to the body wall, running through the cleft in the retractor muscle. The rectal caecum is small. A contractile vessel is not discernible. Two nephridia; each is attached throughout its length.

Habitat:
This sipunculan is found in a wide variety of habitats, from the lower shore to a depth of about 1000 m. It commonly occupies gastropod and scaphopod shells and serpulid tubes but also lives in crevices, amongst coralline algae and in deepwater corals (Lophohelia ). When the introvert is withdrawn the anal shield effectively serves as an operculum blocking the shell aperture or burrow opening. This species does not plug the shell aperture with sediment as is done by Phascolion strombus . The polychaete Syllis cornuta is a frequent cohabitant in shells occupied by A. muelleri .

Distribution:
Widely distributed in the eastern Atlantic from Shetland and Norway to West Africa and eastern Mediterranean. Also reported from scattered localities in the Indo-West Pacific region.