Nucleobranchiates, an order of gasteropod mollusks, so named by De Blainville because the respiratory and digestive organs form a kind of nucleus on the posterior part of the bark; they have been called by subsequent writers heteropoda, the foot being divided into a ventral fin, sucker, and terminal fin. Though they are regarded as the most highly organized of the gasteropods, their form and mode of progression are very different from the type of their class. They are rapid swimmers, found at the surface in mid ocean, moving by their fin-shaped foot and tail, and attaching themselves to seaweed by the sucker. The abdominal portion is small, the anterior parts being greatly developed; the proboscis is large and cylindrical, and the tongue armed with spines; from their transparency, the irregular circulation can be plainly seen. Some have no special breathing organs; the sexes are separate; the nervous centres are widely separated; the eyes and auditory vesicles are well developed. In the firolidce the body is large and the shell small or absent; in the atlantidce, the animal can retire within the shell.
In carinaria, a good specimen of the group, the gills and heart are protected by a small shell; it swims, back downward, by means of its vertically flattened ventral fin; it feeds on minute pteropods and jelly fishes. Many species are found fossil, and the Maclurea, one of the oldest and largest of the Silurian forms, belongs in this order.
Carinaria. - p, proboscis; t, tentacles; b, branchiae; s,shell; f, foot; d, disk.