Decollation.—In certain genera, when the shell becomes adult, the animal ceases to occupy the upper whorls, which accordingly die and drop off, the orifice at the top having meanwhile been closed by a shelly deposit. Such shells are termed decollated. In some land genera decollation is the rule, e.g. in Cylindrella (Fig. [169]), Eucalodium, and Rumina, as well as in many species of the brackish water genera, Truncatella, Cerithidea, and Quoyia. Stenogyra (Rumina) decollata, a common shell in the south of Europe, has been noticed to bang its upper whorls violently against some hard substance, as if to get rid of them.
Fig. 171.—Four stages in the growth of Fissurella, showing how the spire gradually disappears and the marginal slit becomes an apical hole, A, B, C, highly magnified, D, natural size. (After Boutan.)
Fig. 172.—Three stages in the growth of Cypraea exanthema L. (From specimens taken at Panama.)
Special Points in the Growth of Certain Genera.—In the young of Coecum the apex is at first spiral, but as growth proceeds and the long tube begins to form, a septum is produced at the base of the apex, which soon drops off. Soon afterwards, a second septum forms a little farther down, and a second piece drops off, leaving the shell in the normal cylindrical form of the adult (Fig. [170]). The development of Fissurella is of extreme interest. In an early stage it possesses a spiral shell, with a slit on the margin of the outer lip of the last whorl. As growth advances, shelly matter is deposited on both margins, which results in the slit becoming a hole and the spire a mere callosity, until at last they appear to coalesce in the apex of the adult shell (Fig. [171]). The singular formations of Magilus and Rhizochilus have already been described (pp. [75], [76]). Cypraea, in the young stage, is a thin spiral shell with a conspicuous apex. As growth proceeds, the surface of the whorls, which are nearly enveloped by two large lobes of the mantle, becomes overlaid with new layers of shelly matter, until eventually the spire becomes embedded, and ultimately disappears from view (Fig. [172]).
Patella, when young, has a nautiloid shell (see Fig. [45], p. 134), but it is a remarkable fact that we are entirely ignorant, in this commonest of molluscs, of the transition stages which convert the nautiloid into the familiar conical shell. The young shell of Pteroceras is deceptively unlike the adult, and is entirely devoid of the finger-like processes which are so characteristic of the genus (chap. [xiv].).
Among the bivalve Mollusca, Anomia in a young stage is not to be distinguished from Ostrea. Soon a small sinus appears on the ventral margin, which gradually deepens and, as the shell grows round it, forms a hole for the byssus, eventually becoming fixed beneath the umbones (see Fig. [173]). In Teredo the two valves of the shell proper, which is very small, become lodged in a long calcareous tube or cylinder, which is generally open at both ends (see chap. [xvi].). In Aspergillum a somewhat similar cylinder is developed, but the valves are soldered to the tube, and form a part of it, the tube itself being furnished, at the anterior end, with a disc, which is perforated with holes like the rose of a watering-pot. In Clavagella the left valve alone becomes soldered to the tube, while the right valve is free within it (see chap. [xvi].). Fistulana encloses the whole of its shell in a long tapering tube, which is not at any point adherent to the shell.
Fig. 173.—Development of the byssus or the plug-hole in Anomia. (After Morse.)