serves as a useful identification character. It is absent in adults in the following families: Marginellidae, Cypraeidae, Tonnidae, Haliotidae, Acmaeidae, Fissurellidae, Janthinidae, and nearly all of the sea slugs (opisthobranchiates, nudibranchs, bullas, etc.). Some genera lack this organ, such as Oliva and Cypraecassis, although their close relatives, Olivella, Ancilla, Phalium and Cassis possess well-developed opercula. Nearly all Voluta are without the operculum, except our West Indian Music Volute. This is also true of the genera Conus and Mitra whose various species may or may not possess one. In the Alaskan volute, Volutharpa ampullacea Dall, 15 percent have an operculum, 10 percent only traces of the operculigenous area and 75 percent without a trace of either. The presence or absence of this part of the animal is not always a good classificational character.
Many families, genera and species (although not in so many cases as generally believed) possess a characteristic type of operculum. Calcareous or hard, shelly opercula are found in the turban shells (“cat’s eyes” of Turbo), the rissoids, the nerites, and the natica moon shells. The color and sculpturing of these opercula are used for identification purposes. The liotias (Liotiidae) possess a horny operculum which is overlaid by rows of calcareous beads. Among the horny or corneous opercula there are several important and characteristic types which we have illustrated in figure 24.
The radula. The minute teeth or radula (also called the odontophore or lingual ribbon) located in the mouths of all classes of mollusks, except the clams, are so very distinctive in the various families, genera and species that they have been used as a fairly reliable identification criterion. Our present arrangement of the gastropod families is based largely upon the radula, although many other anatomical characters of the animal and shell are equally important. The Greek naturalist, Aristotle, mentioned the radula of snails as early as 350 B.C., but a fuller account was given by the Dutch naturalist, Swammerdam, in the seventeenth century. The Italian malacologist, Poli, was the first to figure the radulae of gastropods, cephalopods and chitons.
The radula is attached to the floor of the buccal cavity or inner mouth and consists of a ribbon-shaped membrane to which are attached many small, fairly hard teeth. The radula ribbon is maneuvered back and forth in somewhat licking fashion as the animal rasps its food. The teeth are arranged in transverse rows on the ribbon (see fig. 6), The number of rows may vary from a dozen (in some nudibranchs) to several hundred. Each transverse row contains a specific number of teeth, depending on the family or group to which the snail belongs. In the taenioglossate snails (many families, including Cypraeidae, Strombidae, Cerithiidae and Littorinidae) there are generally only seven teeth in each row, but each of these teeth has a distinctive shape and a specific number of tiny cusps on its edges. The tooth in the center is called the rachidian or central. Flanking this tooth on each side is a lateral.
