78 American Seas he lis 



serves as a useful identification character. It is absent in adults in the follow- 

 ing families: Marginellidae, Cypraeidae, Tonnidae, Haliotidae, Ac77meidae, 

 Fissurellidae, Janthifiidae, 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 Valuta are without the 

 operculum, except our West Indian Music Volute. This is also true of the 

 genera Conns and Mitra whose various species may or may not possess one. 

 In the Alaskan volute, V olutharpa ampuUacea Dall, 1 5 percent have an oper- 

 culum, 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 calca- 

 reous beads. Among the horny or corneous opercula there are several im- 

 portant and characteristic tvpes 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, al- 

 though 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 some- 

 what 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, includ- 

 ing Cypraeidae, Strovihidae, Cerithiidae and Littoriuidae) tliere 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. 



