CHAPTER IX
THE SHELL, ITS FORM, COMPOSITION AND GROWTH—DESIGNATION OF ITS VARIOUS PARTS
The popular names of ‘shells,’ ‘shell-fish,’ and the like, as commonly applied to the Mollusca, the intrinsic beauty and grace of the shells themselves, resulting in the passion for their collection, their durability and ease of preservation, as compared with the non-testaceous portion,—all these considerations tend to unduly exalt the value of the shell as part of the organism as a whole, and to obscure the truth that the shell is by no means the most important of the organs.
At the same time it must not be forgotten that the old systems of classification, which were based almost entirely on indications drawn from the shell alone, have been strangely little disturbed by the new principles of arrangement, which depend mainly on structural points in the animal. This fact only tends to emphasise the truth that the shell and animal are in the closest possible connexion, and that the shell is a living part of the organism, and is equally sensitive to external influences.
A striking instance of the comparative valuelessness of the shell alone as a primary basis of classification is furnished by the large number of cases in which a limpet-shaped shell is assumed by genera widely removed from one another in cardinal points of organisation. This form of shell occurs in the common limpet (Patellidae), in Ancylus (Limnaeidae), Hemitoma (Fissurellidae), Cocculina (close to Trochidae), Umbrella and Siphonaria (Opisthobranchiata), while in many other cases the limpet form is nearly approached.
Roughly speaking, about three-quarters of the known Mollusca, recent and fossil, possess a univalve, and about one-fifth a bivalve shell. In Pholas, and in some species of Thracia, there is a small accessory hinge plate; in the Polyplacophora, or Chitons, the shell consists of eight plates (see Fig. [2], p. 8), usually overlapping. A certain proportion of the Mollusca have no shell at all. In many of these cases the shell has been present in the larva, but is lost in the adult.
The shell may be
(1) External, as in the great majority of both univalves and bivalves.
(2) Partly external, partly internal; e.g. Homalonyx, Hemphillia, some of the Naticidae, Scutum, Acera, Aplustrum (Figs. [148] and [149]).
Fig. 148.—Aplustrum aplustre L. Mauritius, showing the partly internal shell (S); F, foot; LL, cephalic lappets; TT, double set of tentacles. (After Quoy and Gaimard.)