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Müller, 1846

Larval body transparent with many pigmentiferous amoebocytes. Number of tentacles increase with development, up to 32-42 tentacles present (see table). Yellow pigments are present at the base of the tentacles, around the preoral lobe and near the telotroch. From the 24-tentacle stage onwards, there is one pair of sanguinary globules above the nephridial site and lateral of the large stomach, that has one pair of diverticles. During the 28-tentacle stage and later, definitive (adult) tentacles are formed in two zones under the larval tentacles. At about the 24-tentacle stage, the larva gets ready for metamorphosis, the sanguinary globules merge into one and the body becomes opaque; the size is then about 2 mm. The larva is a slow swimmer and stays in the plankton for 17-20 days.

Maximum length up to 3 mm.

A. branchiata is the larva of Phoronis muelleri Selys-Longchamps, 1903; the name is kept as a technical name and is used for the larva only.

Distribution in the North Sea
The adult Phoronis muelleri occurs in muddy bottoms containing some sand, and is known from various locations in the whole North Sea. The larvaActinotrocha branchiata — is found in the entire North Sea, including the Skagerrak, the Baltic Sea and the Wadden Sea; occurs from July to December, with a maximum in August-September.

World distribution
The adult P. muelleri has a world-wide distribution and has been recorded from (sub)tropical and temperate waters.

Actinotrocha branchiata