The Cephalorhyncha are a group of ecdysozoan animals that traditionally have been classified together with a variety of unusual and lesser known organisms in a larger group variously known as the Aschelminthes, Nemathelminthes, and/or Pseudocoelomata. This larger group is now known to be polyphyletic, and contained several taxa that were not closely related. However, one part of that group -- the priapulids, kinorhynchs, and loriciferans -- appears to form a group in its own right. Now these organisms are grouped with the nematodes and arthropods based on a set of shared characters including the presence of a cuticle and the fact that they periodically shed their cuticle in a process called ecdysis. Some zoologists have suggested an even closer relationship with the nematodes, and call the combination of nematodes and cephalorhynchs the Cycloneuralia.
All cephalorhynchs have a spiny proboscis which can be collapsed inside the head or everted (turned inside out) to gather food using the spines. This is rather like turning a sock inside out, only with the surface of the sock covered with numerous tiny hooks. It is this spiny evertible proboscis that gives the group its name Cephalorhyncha, meaning "beak"-head.
Though cephalorhynchs may abound in marine sands, muds, or gravel, none of the members of the Cephalorhyncha are particularly diverse today, and most are tiny (less than 1 mm long) so they are unlikely to be seen or encountered. Additionally, neither the Kinorhycha nor Loricifera have any fossil record. So you might be asking yourself, "Why is there a whole page about this group on a site devoted to paleontology?" The answer lies in the fossil record of the third group, the Priapulida. Though there are only about 16 species of priapulid worm today, quite a number of fossils of the carnivorous priapulid Ottoia have been found in the Burgess Shale. In fact, priapulids and arthropods are the most common complex animal fossils from this Cambrian locality, and make their appearance in the fossil record as early as most of the first-known members of various major animal groups.
Modern priapulids burrow in marine sediments and many be several centimeters long, though most are microscopic. Larger priapulids primarily inhabit cold waters at hish latitudes, where they hunt for polychaete worms. There are 16 species known to be alive at present.
The Kinorhyncha are microscopic, spiny-headed worms. You can see a greatly enlarged diagram of one at right on this page. Kinorhynchs feed on bacteria and tiny particles associated with sand grains. They are the most diverse group of cephalorhynchs today, with about 150 species. Loricifera is a recently discovered group of animals. Discovered in 1983, these tiny worms have a spiny head, and so resemble the kinorhynchs to which they are related. Only about 10 species have so far been described.
To find out more about these tiny animals try starting with the International Association of Meiobenthologists, whose members study all manner of tiny creatures within sediments (not just cephalorhynchs).