(1372). In many marine Gasteropods, spines and various external processes are found projecting from the outer surface of the shell, the production of which depends upon the shape of the margin of the mantle. Let the reader imagine one of these ornamented shells to be transparent, so as to permit the contained animal to be delineated in situ, as in the annexed sketch of Pterocera (fig. 262); and the collar, which forms the layers of growth, will be found to exhibit fringes or processes precisely resembling those upon the shell itself. But it is only at intervals that, as the growth of the mollusk proceeds, these pallial appendages encase themselves in a calcareous covering, every such interval being distinctly indicated upon the exterior of the shell by the spaces between the successive rows of spinous projections that mark the terminations of so many distinct periods in its formation; so that the number of ridges or rows of spines is, of course, correspondent with the age of the creature within.

Animal of Pterocera in situ.

Fig. 262. Animal of Pterocera in situ.

(1373). Several of the Pectinibranchiate genera are provided with a very complete defence against the assaults of foes that might attack them while they are concealed in their habitations and, in such a posture, necessarily helpless and incapable of resistance. The provision for their protection is sufficiently simple: attached to the posterior extremity of the body, which is the part last drawn into its abode, is a broad horny or calcareous plate (fig. 262,g), called the operculum-, this is of variable dimensions in different species, but always accurately corresponding in shape with the contour of the mouth of the shell. By this elegant contrivance a door is closely fitted to the aperture of its retreat whenever the mollusk retracts itself within its citadel, and, thus defended, it may safely defy external violence of any ordinary description.

(1374). A most remarkable exception to the usual univalve condition of the shells in the Gasteropoda is observable in one solitary genus belonging to the Cyclobranchiate order. In Chiton (fig. 263) we find, instead of a turbinated or shield-like covering formed of one piece, a kind of armour composed of several distinct plates, arranged in a longitudinal series along the centre of the back, and overlapping each other like the tiles of a house.

Chiton: A, ventral; B, dorsal aspect.

Fig. 263. Chiton: A, ventral; B, dorsal aspect.

(1375). In these curious animals the whole back is invested with a dense leathery mantle of an oval form, and considerably more extensive than the cavity containing the viscera. Where not covered by the calcareous laminae, the exterior of the mantle forms a broad edge variously sculptured in different species: but along its central part the shelly plates, generally eight in number, are partially imbedded in its substance; being, no doubt, secreted by the surface whereunto they are attached. These mollusks, notwithstanding the singularity of their covering, which almost reminds us of the armour of many Articulata, in their internal anatomy conform exactly to the type of structure common to the Gasteropod orders, and offer no peculiarities of organization worthy of special notice.

(1376). Feeble and languid as are the sluggish movements of these creatures, they nevertheless present to the eye of the anatomist a type of organization considerably superior to any that we have had an opportunity of considering in such forms of the Heterogangliata as have been described in the preceding chapters. From the superiority of their mode of progression, it is evident that they are adapted to enjoy a less limited intercourse with external objects than even the most highly gifted of the burrowing Conchifera; and accordingly we find in them a nervous system exhibiting a more complete development, senses of a higher character, and, in the organization of their internal viscera, a complexity of parts such as has not heretofore fallen under our notice - every indication, in fact, that they are animals of a higher grade and more elaborate structure. The Gasteropoda, for instance, exhibit a distinct head, in which is lodged a supra-oesophageal ganglion of large proportionate size; and upon the head are found retractile instruments of sensation of peculiar structure, and not unfrequently perfectly-formed organs of vision.

(1377). Let us, however, select one species for particular description; and after having become acquainted with the details of its anatomy, we shall be better prepared to examine such modifications of the various organs as are found in other orders destined to exist under different circumstances.

(1378). The common Snails (Helix) are well known as far as relates to their external appearance; and, insignificant as they might be thought by those unacquainted with their habits, they not unfrequently become formidable pests to the horticulturist, from the ravages caused by their voracity. On examining a Snail more attentively, we find its body partially enclosed in a thick muscular envelope composed of transverse and longitudinal fibres, which, being unsupported by any skeleton, allows the shape of the animal to vary at pleasure, as it is shortened or elongated by the contractions of the muscles composing it. The foot, or ventral disk, is equally composed of an interlacement of muscular fibres, and not only forms an extensive sucker, but, by the successive action of various portions of its substance, a slow and gliding progressive motion is produced.

(1379). From the head of the Snail when its body is expanded, as when in the act of seeking food, four tentacula are protruded (fig. 261, c, d), which, besides being exquisitely sensitive organs of touch, carry, at the extremities of the superior pair, two minute but perfect eyes. When the creature is at rest, the tentacula as well as the eyes are retracted into the visceral cavity, by a mechanism hereafter to be noticed. A large proportion of the viscera is enclosed in a turbinated calcareous shell, of sufficient capacity to allow the whole body of the animal to be withdrawn from observation and lodged in its interior.