Priapulids are worm-like pseudocoelomate invertebrates that appear in the fossil record some 500 million years ago in the Cambrian Period. They have survived virtually unchanged up to the present and so qualify as living fossils. There are only 16 living species of the phylum Priapula, but there were many more species and individuals back in the Cambrian. In fact they were, with the arthropods, the most advanced animals alive at that time and possibly the top predators of their ancient ecosystems.
Their closest relatives are probably the Kinorhynchs and the Loriciferans. Some taxonomists separate these three groups into different phyla while others lump them in a single phylum, the Cephalorhyncha. Cephalorhyncha means beak-head or nose head and all three groups have a proboscis that can be everted or turned inside out. This technique is used to capture and hold food items with the assistance of rows of spines around the head. Priapulids are carnivorous. Smaller species of Priapula eat bacteria while the larger species eat small invertebrates such as polychaete worms and perhaps even other priapulids.
Priapulids are very small, most being only a few millimeters long and the largest, cold-water species reaching a maximum of a few centimeters in length. They live in the sediments of the ocean benthos where they burrow in sand, mud or gravel and in some areas they are quite numerous. They are most common in cold northern waters.
Priapulid bodies are divided into three parts: the proboscis, a trunk and a short tail. They are usually clear or white but a few are orange or pink in colour. They look enough like a certain appendage to have gained the common name of penis worm. They have muscles which they can use to push their way through the sediments to catch their prey. They also have simple nervous systems, but no external organs for sight or hearing so they probably depend on touch to find their prey. They have a simple one way gut consisting of a pharynx, an esophagus, an intestine and anus. There is a body cavity but it is not a true coelom because it is not lined with mesodermal cells. There is a simple branched respiratory organ at the end of the body.
Priapulids, like Kinorhynchs, nematodes and arthropods, have a hard surface covering or cuticle and this has to be shed periodically for the priapulids to grow. This process is called ecdysis.
Priapulids have separate sexes but little is known about their reproductive behaviour. Fertilisation is probably external.
Most people will never see a living Priapulid but fossil priapulids may tell us more about the evolution of multicelled animal forms during the Cambrian period.
References: http://www.ucmp.berkeley.edu/phyla/ecdysozoa/cephalorhyncha.html
http://www.arctic.uoguelph.ca/cpl/organisms/inverts/marine_inverts/priapulids.htm
http://en.wikipedia.org/wiki/Priapulida
