Priapulids

Anatomy

Priapulids are cylindrical worm-like animals, ranging from about 0.2 to 39 centimetres (0.08 to 15.35 inches) long, with a median anterior mouth quite devoid of any armature or tentacles. They show both radial and bilateral symmetry. The gonads, protonephridia and ventral nerve cord are bilateral, while the introvert, pharynx and brain show radial symmetry, which appears to be a secondary trait. The larvae also show internal and external characteristics of radial symmetry. The adult body is divided into a main trunk or abdomen and a somewhat swollen proboscis region ornamented with longitudinal ridges. In addition, it is ringed and often has circles of spines, which are continued into the slightly protrusible pharynx. Family Priapulidae have species with a tail or a pair of caudal appendages. A slender tail or tail filament is also found in family Tubiluchidae. Appendages are absent in the remaining families. The body has a chitinous cuticle that is moulted as the animal grows. Members of the family Chaetostephanidae also secrete a gelatinous tube, open in both ends, which they live in.