Fig. 160.—Monstrosities with two apertures: A, Cylindrella agnesiana C. B. Ad., Jamaica; B, Littorina littorea (from specimens in the British Museum).
Fig. 161.—Cornucopia-shaped monstrosity of Helix aspersa, from Ilfracombe. (British Museum.)
Composition of the Shell.—The shell is mainly composed of pure carbonate of lime, with a very slight proportion of phosphate of lime, and an organic base allied to chitin, known as conchiolin. The proportion of carbonate of lime is known to vary from about 99 p.c. in Strombus to about 89 p.c. in Turritella. Nearly 1 p.c. of phosphate of lime has been obtained from the shell of Helix nemoralis, and nearly 2 p.c. from that of Ostrea virginica. The conchiolin forms a sort of membranous framework for the shell; it soon disappears in dead specimens, leaving the shell much more brittle than it was when alive. Carbonate of magnesia has also been detected, to the extent of ·12 p.c. in Telescopium and ·48 p.c. in Neptunea antiqua. A trace of silica has also occasionally been found.
When the shell exhibits a crystalline formation, the carbonate of lime may take the form either of calcite or aragonite. The calcite crystals are rhombohedral, optically uniaxal, and cleave easily, while the aragonite cleave badly, belong to the rhombic system, and are harder and denser, and optically biaxal. Both classes of crystal may occur in the same shell.
Two main views have been held with regard to the formation and structure of the shell—(1) that of Bowerbank and Carpenter, that the shell is an organic formation, growing by interstitial deposit, in the same manner as the teeth and bones of the higher animals; (2) that of Réaumur, Eisig, and most modern writers, that the shell is of the nature of an excretion, deposited like a cuticle on the outside of the skin, being formed simply of a number of calcareous particles held together by a kind of ‘animal glue.’ Leydig’s view is that the shell of the Monotocardia is a secretion of the epithelium, but that in the Pulmonata it originates within the skin itself, and afterwards becomes free.[335]
According to Carpenter, when a fragment of any recent shell is decalcified by being placed in dilute acid, a definite animal basis remains, often so fine as to be no more than a membranous film, but sometimes consisting of an aggregation of ‘cells’ with perfectly definite forms. He accordingly divides all shell structure into cellular and membranous, according to the characteristics of the animal basis. Cellular structure is comparatively rare; it occurs most notably in Pinna, where the shell is composed of a vast multitude of tolerably regular hexagonal prisms (Fig. [162] B). Membranous structure comprises all forms of shell which do not present a cellular tissue. Carpenter held that the membrane itself was at one time a constituent part of the mantle of the mollusc, the carbonate of lime being secreted in minute ‘cells’ on its surface, and afterwards spreading over the subjacent membrane through the bursting of the cells.
The iridescence of nacreous shells is due to a peculiar lineation of their surface, which can be readily detected by a lens. According to Brewster, the iridescence is due to the alternation of layers of granular carbonate of lime and of a very thin organic membrane, the layers very slightly undulating. Carpenter, on the other hand, holds that it depends upon the disposition of a single membranous layer in folds or plaits, which lie more or less obliquely to the general surface, so that their edges show as lines. The nacreous type of shell occurs largely among those Mollusca which, from other details in their organisation, are known to represent very ancient forms (e.g. Nucula, Avicula, Trigonia, Nautilus). It is also the least permanent, and thus in some strata we find that only casts of the nacreous shells remain, while those of different constitution are preserved entire.
Porcellanous shells (of which the great majority of Gasteropoda are instances) usually consist of three layers, each of which is composed of a number of adjacent plates, like cards on edge. The inclination of the plates in the different layers varies, but that of the plates in the inner and outer layer is frequently the same, thus if the plates are transverse in the middle stratum, they are longitudinal in the inner and outer strata, and, if longitudinal in the middle, they are transverse in the other two. Not uncommonly (Fig. [163] B) other layers occur. In bivalves the disposition and nature of the layers is much more varied.
