Ocnus

Forbes, 1841

25 species

ten tentacles. Calcareous ring simple, without forked tail. In the skin are cups as well as knobbed plates in two forms, and in two layers. The plates usually construct a firm skin structure (skeleton). The radii have no growths. Calcareous deposits cups and knobbed plates of two types. (Pawson, 1970)

The shape is variable, cucumber to spindle shape to egg shape; mainly spindle shape with angular protruding radii. The feet are confined to the radii, the interradii are bare. The feet are one to two rowed in the radii, in part alternate; they are, indeed, retractable. Mostly, however, they are relaxed (extended) and easily appear rigid and not retractable. The calcareous ring is slender and high, has no forked tail; the hind section is strong, the hind edge of the calcareous ring is deeply incised in the radialia and interradialia. There are 10 tentacles. The 2 ventral ones are smaller. In the skin there are cups to the surface, or strongly spiked plates, or network plates. Below that in the skin are oval plates with small perforations in varying number, thickly covered with high round bumps. (Panning, 1971)

This genus has become a grab bag of disparate species and is in need of thorough revision. When this is done, O. farquhari and O. sacculus will probably be congeners in a new genus. (Pawson, 1983)