The anterior end of the shell is that end which is in front when the animal is crawling. The aperture, the siphonal canal (when present), the head and the tentacles of the mollusk are at this end. The posterior end is the opposite, where the apex and nuclear whorls are located, hence it is sometimes referred to as the apical end. When we speak of the posterior side of a rib or a bar of color we mean the side nearest to the apex or away from the anterior end of the mollusk. The total distance between the two ends of the shell is known as the length, although this measurement is often called the height.
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Figure 23. Parts of the gastropod shell.
The aperture of the shell is the hole or space at the end of the body whorl into which the mollusk can withdraw itself. The edge of the body whorl which borders the aperture is known as the lip (sometimes called the peristome in technical works). Sometimes the lip is thickened greatly or flaring like an old-fashioned blunderbuss. Any startling development of the lip is generally a sign of adulthood. If the lip thickens into an unusually large, rounded, sharp rib it is known as a varix. Varices may be produced at various
