These primitive mollusks have a linear series of perforations for the exhalant mantle cavity currents. Slits in the mantle and shell facilitate the excurrent water leaving the mantle cavity. This is a primitive solution to the sanitation problems caused by torsion (having the mantle cavity with the anus, nephridiopores, and gonopores located over the head).

Genetic diversity. The shells in any population of Umbonium are highly polymorphic.

Commonly called "button shells" because of their size and colors, these primitive mollusks inhabit sandy littoral habitats in the Indo-Pacific. Note the multispiral operculum.


John B. Burch (author), Mollusk Division, Museum of Zoology, University of Michigan-Ann Arbor.


bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.


animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature