Let us now take a good example of a gasteropod and locate its various organs; at the same time we may use the occasion to refer to more important modifications of these organs which will be encountered later in the various genera. The most available gasteropod on the east coast of the United States for this purpose, is /Fulgur/, both on account of its large size and its abundance. /Buccinum/ may be used if the student is north of Cape Cod and therefore unable to secure a good living specimen of /Fulgur/; the anatomical differences between the two are slight. Note the siphon protruding forward from a notch in the shell. This consists merely in an elongation of a fold of the mantle, which is held in a manner to constitute a tube, through which [pg331] the animal draws in water to supply the gills, lying just back of the siphon and concealed under the mantle in the cavity already spoken of as the mantle cavity. With the exception of that portion of the mantle which constitutes the siphon, no part of its margin, it will be observed, can be seen, the edge of the shell completely hiding it from view. In some genera the siphon is exceedingly long—in fact, longer than the body of the animal. In these cases the mollusk is generally fond of burying itself in the sand and maintaining communication with the world above only by means of this long, fleshy tube. In some genera the siphon is protected by an elongation of the shell, as in /Fasciolaria/ (Plate LXXVI). An extreme example of this is given in the figure on page 343, which represents a species of /Murex/, where the siphon is not only protected by the shell, but the shell itself is further protected by the double row of sharp spines with which it is beset. [Illustration: /Buccinum undatum/: /si/, siphon.] There are some genera which possess no siphons at all, in all of which cases there is no notch in the shell opening, the aperture being round, as in /Natica/ and in /Littorina/ (pages 368, 371). The mollusks of this type make a fold in the forward part of the mantle which serves as a siphon. Such a muscular folding in the mantle would be technically called a /functional siphon/. On the posterior dorsal side of the foot is attached a horny plate, of concentric structure about a central or subcentral point or nucleus. This is called the /operculum/, and serves to close the entrance of the shell when the animal is withdrawn, the object no doubt being protective. The opercula vary to a great extent in the different genera; in some cases they are calcareous, in others cartilaginous. When the aperture of the shell is very large it often happens that the operculum does not entirely close the opening, as in /Fulgur/, but in /Buccinum/ it is a close fit. The operculum is entirely wanting in the /Pulmonata/, one of the [pg332] large orders of the /Gasteropoda/. The figures represent some of the commoner types of opercula.