As we should expect from the mode of its growth, the shell throughout all the Conelli ferous class is composed of earthy matter, cemented together by an animal substance easily separable by the action of acids. In the porcellaneous shells the animal matter exists in much less quantity than in those of a fibrous texture ; in the former, indeed, Mr. Ilatehett found that when the carbonate of lime, of which the earthy portion is almost entirely formed, is dissolved even by very feeble acids, little or no vestige of any membranous struc ture could be perceived, nor indeed could any be detected, but by the small portion of animal coal which was formed when these shells had been exposed for a short time to a low red heat ; in others however, as the I'atellT, a sub stance was left untouched by the acids which had the appearance of a yellowish transparent jelly, by means of which the earthy matter had been, as it were, cemented together.
On examining minutely the mechanical al rangement of the layers of which these shells are composed, it is found to vary in different kinds, and from this circumstance the fossil conehologist may derive important information in examining mutilated remnants sometimes so plentifully met with in calcareous strata. The simpler shells (Patella, Fissurella) are formed of very thin, compact, and parallel layers, whilst in others three distinct strata of fibres, each of which assumes a different direction, may be observed. The fibres composing the external layer are disposed perpendicularly to the axis of the shell. In the middle stratum the fibres are placed obliquely and are slightly twisted, hut so arranged that each meets at an obtuse angle the extremity of one of the fibres composing the outer layer, and in the internal stratum they again assume a perpendicular direction. Such a disposition of the fibres, which is met with in all Siphonibranchiate shells, is eminently calculated to resist ex ternal violence in whatever direction it may act, and greatly contributes to the solidity of the whole fabric.
Opercutum.—Many of the spirivalve Gaste ropoda, especially such as are aquatic, are provided with a calcareousplate, which is placed upon the posterior surface of the body, and closes accurately the mouth of the shell, when the animal is retracted within it. The texture of the operculum is sometimes horny, but it is more frequently calcareous and of a stony hardness, its contour being accurately adapted to the orifice. It is composed of parallel fibres disposed perpendicularly to the base of the shell, and deposited in successive layers around an axis, so as to give to the whole structure the appearance of a solid spirivalve, as may readily be seen on removing it from the animal and examining its inner surface. This has been looked upon by some zoologists as analogous to the second valve of bivalve to which, but for its want of a ligamentous attachment, it certainly bears a distant resemblance.
The deciduous operculum of terrestrial Gasteropoda, or epiphragma, as it is usually called, is a widely different structure, being merely an inspissated secretion, with which, during the period of hybernation, the entrance to the shell is closed ; and on removing the outer plate, not unfrequently a second or even a third similar membrane will be found within, forming additional safeguards against intrusion or the vicissitudes of temperature.
During the progressive growth of the shell the animal contained within it necessarily changes its original position, advancing gra dually as the body enlarges from the earliest formed spires towards the aperture, as may easily be proved by sawing off the apex of a spirivalve shell containing the living animal. This circumstance is remarkably conspicuous in some of the Bulimi decallatus), enabling the occupant, as it grows, to break off the turns of its spire which first contained it, so that at the latter period of its life it does not retain any part of its original shell. The mode in which this advancement is effected is a subject of much curiosity, as it involves a power of detaching the muscles connecting the creature with its abode, from the place where they were originally fixed, and forming a new connexion with the shell; but whether this is effected by the removal of the original fibres and the production of others more ante riorly, as is believed by some, or whether, as is more probably the ease, the creature has a power of changing the attachment of its re tractor muscle at pleasure, is still a matter of uncertainty.
Organs of digestion.—We shall not be sur prised to find that in a class so extensive, and composed of individuals living in such diver sified circumstances, the alimentary organs are much modified in form in different species, according to the nature of the food with which they are nourished.
Mouth.—In most instances the mouth pre sents the appearance of a retractile proboscis, which can be protruded or shortened at the will of the animal, but unprovided with jaws or any apparatus for mastication ; it is in such cases a muscular tube, formed of longitudinal fibres prolonged from the common parietes of the body, and of circular muscles, the former serving for the retraction of the organ, the latter causing its elongation by their successive action ; by means of this simple structure every movement requisite for the prehension of food is effected. At the bottom of the tube is a narrow vertical aperture, the edges of which are slightly cartilaginous, and behind this is the tongue armed with spines variously dis posed ; the aliment therefore, having been forced by the contractions of the proboscis through the aperture at its termination, is re ceived by the tongue, and by the aid of the latter organ is propelled into the oesophagus without mastication or any preparatory change.