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Monoplacophorans are small and limpet-like, having a single, cap-like shell. Some organs
(kidneys, heart, gills) are repeated serially, giving rise to the now falsified hypothesis that
they may have a close relationship with segmented organisms such as annelids and arthropods.
In fossil monoplacophorans, the aperture varies in shape from almost circular to pear-shaped. Shell height is also variable and ranges from relatively flat to tall. The monoplacophoran animal has a poorly defined head with an elaborate mouth structure on the ventral surface. The mouth is typically surround by a V-shaped, thickened anterior lip and post-oral tentacles; post-oral tentacles come in a variety of morphologies and configurations. Below the head lies the semi-circular foot. In the pallial groove, between the lateral sides of the foot and the ventral mantle edge, are found 5 or 6 pairs of gills (there are fewer in very tiny taxa). |
In Recent and fossil limpet-like monoplacophoran shells the apex is typically positioned at
the anterior end of the shell, and in some species the apex actually overhangs the anterior edge of
the shell.
Internally the monoplacophoran is organized with a long, looped alimentary system, two pairs of gonads, and multiple paired excretory organs (4 of which also serve as gonoducts). A bilobed ventricle lies on either side of the rectum and is connected via a long aorta to a complex plumbing of multiple paired atria that in turn are connected to the excretory organs. The nervous system is ladder-like and has weakly developed anterior ganglia. Paired muscle bundles enclose the visceral mass. Large dorsal paired cavities are extensions of glands associated with the pharynx. The monoplacophoran radula is docoglossate: each row having a central tooth, three pairs of lateral teeth, and two pairs of marginal teeth. |
