GASTEROPODA, (vacrriy, venter, wovc, by Ferussac, of which, as svell as of the 'vs; Eng. Gasteropods; Fr. Gastiropodes ; systems of other zoologists, an outline is con Germ. Baucylisser ; lllopnsro Repentia, tained in the following table.
Poli.) Definition.—An extensive class of the Order I. NUDIBRANCIIIA'fil,* (Cuv.) luscous division of the animal kingdom dis- Syn. Polybranchiata,f and genus Doris, tinguishcd by the structure and position of their Blainville ; Gesteropodcs Dermobranehcs,1 Du locomotive apparatus, which consists of a mus- merit ; Gasteropades Tritoniens, Lamarck.
cular disc attached to the ventral surface of the In these the bmnehite are symmetrical, body, serving either as an instrument by means sliming a variety of forms, but always placed of which the animal can crawl, or in rarer upon some part of the back, where they are instances compressed into a muscular mem- unprotected by any covering ; the animals may brane useful in swimming. be provided with a shell or naked, but they Characters af the class.—Body soft, enclosed are all hermaphrodite with mutual copulation, in a muscular covering, which, from its contrae- and marine.
tility in every direction, produces great variety 1st Sub-order, Anthobranehiata,§ Goldfuss; in the external form of the animal : the back is Cyclobranehiata,11 Blainville.
covered with a mantle of greater or less extent, 1st lam. Doris.
which in most of the genera secretes a shell 2d Sub-order, Polybranehiata, Blainville.
either enclosed within its substance, or, as is 2d Fam. Tritonia, fig. 173.
more frequently the case, external and 3d Fam. Glaucus, fig. 174.
ciently large to conceal and protect the whole body, in which case it is often provided with an operculum capable of closing its orifice when the animal is lodged within it. The head is anterior, distinct, and generally furnished with two, four, or six tentacles, which are placed above the oral aperture, and merely serve as instruments of touch. The eyes are two in number, and are placed sometimes on the head itself, but more generally at the base, at the side or at the extremity of the tentacles; they are always very small, and not unfre qucntly wanting. The muscular disc which is
subservient to locomotion is called the foot, and is generally broad and fleshy, forming a powerful sucker, bnt in some instances it takes the shape of a deep furrow, or is compressed into a vertical lamella. The respiratory appa ratus varies in structure ; in some genera it is composed of vascular ramifications which line a cavity into which the respired medium is freely admitted. Others are provided with branchix, adapted to the respiration of water, variously disposed upon the exterior of the body, or concealed internally. The heart generally consists of an auricle and ventricle, and is systemic, or, in other words, receives the blood from the organs of respiration, and propels it through the body. The sesual organs vary in their structure in different orders ; in the greater number each individual is possessed both of an ovigerous and impreg nating apparatus, but copulation is essential to fecundity in many the sexes are distinct, and some are hermaphrodite and self-impreg nating. Some species are terrestrial and others aquatic.
In separating the Gasteropoda into orders, the naturalist finds in the position and structure of the branchial apparatus a character suffi ciently obvious ; and as the arrangement of these organs is modified by the circumstances of each individual, and is generally in relation with the peculiarities met with in the internal organization of the animal, the branchite are at present universally referred to as affording a convenient basis of classification. We shall in this article follow the arrangement adopted Order II. INPEROBItANCHIA7'A, (Cuv. and Blain.) Syn. Gast. Dermabranches, Dumeril ; Gast. Phyllidiens, Lamarck.
In the Inferobranchiate Gasteropods the branchite are arranged under the inferior border of the mantle on both sides of the body, or upon one side only : the mantle sometimes contains a calcareous lamella. All the genera are hermaphrodite with reciprocal impregnation, and marine.