The ambulacral pattern of the Caryocystitida differs from that of all other primitive echinoderms. Phylogenetically the number of ambulacra increased from two to three and finally to four. In the Blastozoa three primary ambulacral patterns are recognised: a pentaradiate, a triradiate and a biserial. The pentaradiate pattern is present in several classes, the triradiate is restricted to the Hemicosmitida and the bilateral pattern is confined to the Caryocystitida. There appears to be considerable differences between the three superfamilies of the class Rhombifera. These differences in primary patterns may have important taxonomic significance.