Sand horseshoe worm

Phoronis psammophila constructs and lives in a rigid, chitinous tube about 10 cm long, incorporating sand grains and detritus. The extended worm is up to 19 centimetres long but it can contract down to about one fifth of this length. The body is pinkish and is divided into two sections. The anterior part, the mesosome, has a cavity, the mesocoel, that extends into the tentacles and keeps them rigid by hydrostatic pressure. The mesosome bears the lophophore, a specialist feeding structure which consists of a ring of up to 190 translucent tentacles arranged in a horseshoe-shape encircling the crescent-shaped mouth. The posterior and larger body section is the metasome and contains the metacoel. It is swollen at the base into an ampulla which may provide grip inside the tube. The gut is U-shaped and extends from the mouth to the ampulla before doubling back to the anus which is situated just below the mouth. The gonads are located in the metacoel. There are two blood vessels running along the ventral and dorsal sides of the body with capillaries in the tentacles. These are made easily visible by the haemoglobin in the red blood cells. There is a single nerve fibre on the left side of the body.

The distribution is cosmopolitan and the type locality is Messina in Italy. P. psammophila is found in the intertidal zone and at depths down to 70 metres. It favours a substrate of fine sand with a moderate silt content and sometimes occurs at densities of up to 20,000 individuals per square metre.