Physical characteristics

Caenogastropod shells are typically coiled and almost every shell form is found within this group, from flat squat shells to globose ones and even long, narrow, tightly coiled ones. A few are limpet-like, and one group, the Vermetidae, has shells that uncoil and look like worm-tubes; in a few caenogastropods, the shell is reduced to an internal remnant in the snail's body. The most pronounced morphological change is in groups such as the Entoconchidae, which are shell-less, worm-like internal parasites of echinoderms.

A major feature of the caenogastropods is modification to the pallial cavity, which contains the gill (or ctenidium) and associated sense organs and openings from the kidneys,

The red volute species (Xenophora pallidula) is a member of the carrier shell family. (Photo by Michael McCoy/Photo Researchers, Inc. Reproduced by permission.)

gonad, and intestine. In the Architaenoglossa, the terrestrial Cyclophoroidea have lost the ctenidium and osphradium, and the pallial cavity has been modified as a lung. In contrast, the freshwater Ampullarioidea have a single left ctenidium in addition to a new structure located on the right side of the pallial cavity that serves as a lung. In the Sorbeoconcha, pallial cavity water flow is driven by the large, ciliated osphradium, as well as by the ctenidium. There is also the formation of an anterior (or inhalant) notch, or siphon, in the shell and the mantle in many taxa. Some taxa also have a posterior notch in the shell that is associated with the exhalant current, and in some this resembles a shell-like tube.

The shell is never nacreous, and an operculum is present in most adults. Apart from members of the Neogastropoda, the radula usually has only seven teeth in each row. The radula of neogastropods has five teeth to one tooth in each row; it is altogether absent in some species.

The caenogastropods include some of the largest and smallest gastropods, ranging in adult size from 0.019-15.7 in (0.5-400 mm). The caenogastropods are also the most colorful of the gastropods and have the most elaborate markings and patterns.

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