|A large retractable head
region, and spiny or scaly trunk
|An alimentary system with
mouth surrounded by spines or hooks
|Larva enclosed in cuticular
|A nervous system with ventral
|Separate sexes and external
At present only about
16 species of Priapula have been described, though fossil evidence dates back
to the Cambrian.
Priapulida are up to 15 cm long
and are found in colder waters at a variety of depths from tidal to abyssal.
Their body cavity has a
mesodermal lining, so they can be regarded as coelomate; however the lining is
unlike that of other coelomates. The cuticular trunk is moulted periodically.
Circular and longitudinal muscles exert the pressure required to evert the head
region (see Priapulus bicaudatus right). The larvae move through the sediment using their head
region as an anchor, but adults are rarely good burrowers.
They are predators feeding on slow-moving animals, especially worms.