[Illustration: Various forms of opercula. /Turbo/ (/Sarmaticus/). /Turbo/ (/Callopoma/). /Livona./ /Ampullaria./ /Natica./ ] The opercula have been utilized a great deal in the classification of the /Gasteropoda/, and especially in some groups that have become terrestrial in habit. Often, when shell-characters utterly fail to give a clue to the proper generic position of a new or rare mollusk, the operculum may be relied upon to indicate it. [Illustration: Various forms of opercula. /Pyrula./ /Purpura./ /Littorina./ /Aulopoma./ /Torinia./ /Neritopsis./ /Strombus./ /Conus./] TENTACLES Two tentacles project like fleshy horns from the sides of the head. These are not retractile, as in the land-snails, and their office appears to be that of sensory organs. In both /Fulgur/ and /Buccinum/ the tentacles are comparatively short and stout, and have expanded bases; in many other genera they are long and slender. Almost without exception the /Gasteropoda/ possess tentacles. [pg333] EYES In /Fulgur/ the eyes are situated near the bases and upon the outer sides of the tentacles; in /Buccinum/ they are extremely small and are mounted about midway between the bases and the tips of the tentacles. This is the usual position for the eyes in the marine /Gasteropoda/. Only in the pulmonates, or land-snails, are the eyes placed at the summit of the tentacles. Sight does not appear to be a very important sense in the gasteropods, although the eye in some instances becomes a highly developed organ. A number of forms that have the habit of burrowing deep into the sand or mud are quite destitute of visual organs, and certain cave-dwelling land-shells have entirely lost the sense of vision. MOUTH AND PROBOSCIS The mouth will easily be seen in both /Fulgur/ and /Buccinum/. It is a small slit with slightly thickened, fleshy lips, and is placed on the lower forward portion of the head. In the case of both of these genera the mouth is situated at the end of a proboscis. It will be noted that /Fulgur/ has a comparatively long and stout proboscis, or snout, which is not retractile. /Buccinum/ has a smaller one. It is fairly safe to assume that all gasteropods that possess a proboscis are carnivorous. This organ is greatly developed in some genera, where it actually exceeds in length the rest of the animal. In such instances, no doubt, the creature is enabled to reach the interior portions of the shells of its victims and to devour every vestige of their flesh, for the mouth is most conveniently placed just at the end of this long, trunk-like organ. The genera which have no proboscis are, almost without exception, vegetable feeders.