Fig. 155.—Various forms of the internal plate in Capulidae: A, Calyptraea (Mitrularia) equestris Lam., E. Indies; B, Crucibulum scutellatum Gray, Panama; C, Ergaea plana Ad., and Reeve, Japan; D, Galerus chinensis L., Britain; E, Crepipatella dilatata Lam., Callao; F, Trochita maculata Quoy, N. Zealand; G, Crepidula fornicata Lam., N. America.
Some genera of the Capulidae, in which the shell is of a broadly conical form or with scarcely any spire, develop an internal plate or process which serves the purpose of keeping the animal within the shell, and does the work of a strong attachment muscle. In Mitrularia this process takes the form of a raised horse-shoe; in Crucibulum it is cup-shaped, with the edge free all round; in Galerus, Ergaea, Crepipatella, and Trochita we get a series of changes, in which the edge of the cup adheres to the interior of the shell, and then gradually flattens into a plate. In Crepidula proper this plate becomes a regular partition, covering a considerable portion of the interior (Fig. [155] G). Hipponyx secretes a thin calcareous plate on the ventral surface of the foot, which intervenes like an operculum between the animal and the substance to which it adheres.
Sinistral, or Left-handed Shells.—The vast majority of univalve spiral shells are normally dextral, i.e. when held spire uppermost, with the aperture towards the observer, the aperture is to the right of the axis of the spire. If we imagine such a shell to be a spiral staircase, as we ascended it we should always have the axis of the spire to our left.
Sinistral or ‘reversed’ forms are not altogether uncommon, and may be grouped under four classes:—
(1) Cases in which the genus is normally sinistral; (2) cases in which the genus is normally dextral, but certain species are normally sinistral; (3) cases in which the shell is indifferently dextral or sinistral; (4) cases in which both genus and species are normally dextral, and a sinistral form is an abnormal monstrosity.
Fig. 156.—Fulgur perversum L., Florida. ½.
Fig. 157.—Illustration of the gradation of forms in Ampullaria between a dextral (A) and an ultra-dextral species (F).
In all cases of sinistral monstrosity, and all in which a sinistral and dextral form are interchangeable (sections 3 and 4 above), the position of the apertures of the internal organs appears to be relatively affected, i.e. the body is sinistral, as well as the shell. This has been proved to be the case in all specimens hitherto examined, and may therefore be assumed for the rest. The same uniformity, however, does not hold good in all cases for genera and species normally sinistral (sections 1 and 2). As a rule, the anal and genital apertures are, in these instances also, to the left, but not always. In Spirialis, Limacina, Meladomus, and Lanistes the shell is sinistral, but the animal is dextral. This apparent anomaly has been most ingeniously explained by Simroth, Von Ihering, and Pelseneer. The shell, in all these cases, is not really sinistral, but ultra-dextral. Imagine the whorls of a dextral species capable of being flattened, as in a Planorbis, and continue the process, still pushing, as it were, the spire downwards until it occupies the place of the original umbilicus, becoming turned completely ‘inside out,’ and we have the whole explanation of these puzzling forms. The animal remains dextral, the shell has become sinistral. A convincing proof of the truth of this is furnished by the operculum. It is well known that the twist of the operculum varies with that of the shell; when the shell is dextral, the operculum is sinistral, with its nucleus near the columella, and vice versâ. In these ultra-dextral shells, however, where it is simply the method of the enrolment of the spire that comes in question, and not the formation of the whorls themselves, the operculum remains sinistral on the apparently sinistral shell.