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Show 3~6 MOLLUSCA. its edge, sometimes because the mantle is entirely detached from the body along this edge, and sometimes because it is perforated there. It sometimes happens that the margin of the mantle is pro-longed in a canal, in order to allow the animal to seek the ambient element without protruding its head and foot from its shell. The latter, in this case, has also an emargination orcanal in its edge, for the purpose of lodging that of the mantle. The canal, consequently, in ordinary species, is on the left; and in the reversed, on the right. The animal, however, being very flexible, frequently changes the position of the shell, and most commonly when there is an ernargination or canal, it directs the latter forwards, which throws the spine behind, the columella to the left, and the opposite margin to the right. It is the contrary in the reversed, for which reason their shell is said to be contorted to the left. The aperture of the shell, and consequently the last whorl, are more or less large, in proportion to the other whorls, as the head or foot of the animal, which is constantly protruding from and retracting within them, is more or less voluminous in proportion to the mass of the viscera which remain fixed in the shell. · This aperture is wider or narrower in proportion to the greater or less degree of thickness of these same parts. The aperture of some shells is narrow and long-this is because the foot is thin, and becomes folded in order to enter. Most of the aquatic Gasteropoda, with a spiral shell, have an operculum, a part sometimes horny, sometimes calcareous, attached to the posterior part of the foot, which closes the shell when its occupant is withdrawn into it and folded up. In some of the Gasteropoda the sexes are separate; others are hermaphrodite; some of the latter possess the faculty of self impregnation, while others require a reciprocal coitus. Their organs of digestion vary as much as those of respira· tion. This class is so numerous that we have been compelled to GASTEROPODA. 327 divide it into a certain number of orders which . h fi d d h · · ' \\ e ave oun • e upon t e positiOn and form of the branchire. The PULMONEA Re~pire the natural air in a cavity, the narrow orifice of which they open and shut at pleasure. Some of them have no she11, others have .one which is even frequently turbinated but the operculum IS always wanting. The ' NUDIBRANCHIATA H. ave ~n o shell, and are fm·nished with nake·d br,a nc h"1 re, of va-rzous •Orms, on some part of their back. The INFEROBRANCHIATA ' Simila: in other respects to the N udibranchiata have their branchire on the margin of their mantle. The ' TF.CTIBRANCHIA T A Have bran.chire. on the back and side, covered by a lamina of the mant1e, whiCh. generally contains a shell more or Jess dev, eloped, or sometimes only involved in a recurved margin of t 1e foot. lTh~se four orders are hermaphrodites, requiring a reciproca coitus. The HETEROPODA Have their branchire on the back, where they form a trans-t vherse. range .of small panaches, protected, as well as part of e VIsce. ra ' m so me speci· es, b Y a symmetrical sheiJ. They are particularly distinguished however by the foot which is compressed · t h" · ' 1 m o a t m, vertical fin, on whose margin is fre ~~elnt y observed a small cup, the only vestige of the horizon~ oot of the rest of the class. In the PECTINIBRANCHIAT A The sexes are d h . C • separate ; t e respiratory organs almost always onsist of b ranc hI' re, composed of JameJlre, united in the form \ |