It will be noticed that in the above general description of univalve shells we have introduced a number of technical terms which are printed in italics, and this we have done advisedly, for the employment of these terms is a very great convenience when giving descriptions of individual shells, and we shall use them somewhat liberally in noting the distinguishing characteristics of the families and genera; but before entering into this portion of our work we must briefly note the general features of the bodies of the _Cephalophora_. [Illustration: FIG. 157.--DIAGRAM OF THE ANATOMY OF THE WHELK, THE SHELL BEING REMOVED _c_, stomach; _e_, end of intestine; _g_, gills; _h_, ventricle of the heart; _a_, auricle; _f_, nerve ganglia; _b_, digestive gland; _ft_, foot; _o_, operculum; _d_, liver] Sometimes these bodies are bilaterally symmetrical, as we have observed is the case with the worms, but more commonly the organs on one side are aborted, while the growth proceeds apace on the opposite side. Thus the animal assumes a spiral form, being coiled towards the aborted side, with the gills and other organs developed on that side only. As a rule this curvature is such that the body takes the form of a right-handed or dextral spiral, as we have already observed in the shells which cover them, the mouth being thus thrown to the right, but sometimes it takes the opposite direction. When one of these animals is extended and creeping, we observe that it has a distinct head, furnished with a mouth below, and tentacles and eyes above; also, if an aquatic species, the gills are more or less prominent. Further, the exposed portion of the body is covered with a leathery mantle, and the animal creeps on a broad, flattened surface which is called the foot. The tentacles or feelers are usually retractile, and, when retracted, are turned outside-in. Each one is provided with a muscle that runs from the body internally to the tip; and, by the contraction of this muscle the tentacle is involuted just in the same way as the finger of a glove could be by pulling a string attached to the tip inside. In addition to these tentacles, and the eyes and mouth previously mentioned, the head is furnished with ear-sacs, which are little cavities, filled with fluid containing solid particles, with nerve filaments distributed in the walls. On the floor of the mouth there is a ribbon, supported on a base of gristle, and covered with numerous minute teeth arranged regularly in rows. The gristle is moved backwards and forwards by means of muscles in such a manner that this 'lingual ribbon' acts like a rasp, and is employed in scraping or tearing away portions of the substance on which the animal is feeding. By this action the teeth are gradually worn away in front, but this is of no consequence, for the lingual ribbon is always growing forwards, the worn material being replaced by new growth behind.