Fig. 176.—Solarium perspectivum Lam., from the under side.
Fig. 177.—Section of Turbinella pyrum L., showing the plicae on the columella and the growth of successive whorls.
The folds or plaits on the columella, which are often characteristic of the genus or even family (e.g. Fasciolariidae, Mitridae, Turbinellidae) are not merely external, but continue down the whole spire (see Fig. [177], which also shows how successive fresh growths have thickened the columella).
The whorls may be wound in a spiral, which is either hollow, as in Solarium, or quite compact, as in Oliva, Terebra, Cypraea, with every possible intermediate grade. This concavity, which varies in depth and width, is known as the umbilicus, and shells are accordingly spoken of as deeply (e.g. most Trochidae and Naticidae), narrowly (e.g. Lacuna, Littorina), or widely (e.g. Solarium) umbilicated. When the spiral is quite flat, as in Planorbis and some Helix, the umbilicus vanishes entirely. Shells in which the whorls are so compactly coiled on an ascending spiral that there is no umbilicus, are termed imperforate.
Fig. 178.—The slit in A, Hemitoma, B, Emarginula, C, Macroschisma, D, Craniopsis, E, Puncturella, F, Fissurella.
The Slit.—Many shells are furnished with a slit in the last whorl, which opens, in most cases, on the outer lip, and is sometimes of considerable depth, at others a mere notch. In the patelliform shells it is always in front of the apex. The function of the slit appears to be mainly anal, the excretory products being thus allowed to escape by a passage of their own, without soiling the clean water taken in by the branchiae. The posterior canal of some Gasteropoda probably performs a similar function. In the adult Fissurella the slit becomes an apical hole (see Fig. [178] F), in the allied genera it is either immediately in front of the spire (Puncturella), or half-way between the spire and the anterior margin (Rimula), or on the margin and well marked (Emarginula), or a mere indentation of the margin (Hemitoma). In Pleurotomaria it is exceptionally long, and is well marked in Bellerophon, Schismope, Scissurella, Murchisonia, and Pleurotoma (where it is sutural). In Haliotis and Polytremaria it is replaced by a series of holes, which are closed up as the animal grows past them. Some of these holes (at least in Haliotis) certainly serve the purpose of admitting water to the branchiae, while others are anal. In Trochotoma there are only two holes, united by a narrow fissure.
The Tubed Land Operculates.—A group of the Cyclophoridae, which is restricted to Further India and the great Malay Islands, has developed a remarkable sutural tube on the exterior of the last whorl, near the aperture, A similar tube, but more obscure, exists in Alycaeus. Several stages in the development of this tube may be noticed, beginning with the elevation of part of the peristome into a simple irregular shelly plate, which is continued, first into a short, and then into a long tube, which becomes soldered to the shell; finally, the tube becomes free, and the anterior part of the last whorl is disconnected from the spire (Fig. [180] A-D).
