GASTEROPODA. 



377 



GASTEROPODA, (yttcrnf, venter, vove, 

 pes ; Kng. Oatteropodti \'r. Oatttropodti ; 



Germ. Bauc/i/'usser ; Mulliuca Reprntia, 

 Poli.) 



Definition. An extensive class of the Mol- 

 luscous division of the animal kingdom dis- 

 tinguished by the structure and position of their 

 locomotive apparatus, which consists of a mus- 

 cular disc attached to the ventral surface of the 

 body, serving either as an instrument by means 

 of which the animal can crawl, or in rarer 

 instances compressed into a muscular mem- 

 brane useful in swimming. 



Character! tift/ieclass. Body soft, enclosed 

 in a muscular covering, which, from its contrac- 

 tility in every direction, produces great variety 

 in the external form of the animal : the back is 

 covered with a mantle of greater or less extent, 

 which in most of the geneia secretes a shell 

 either enclosed within its substance, or, as is 

 more frequently the case, external and suffi- 

 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- 

 qurntly 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- 

 atus varies in structure ; in some genera it is 

 omposed of vascular ramifications which line 

 i cavity into which the respired medium is 

 freely admitted. Others are provided with 

 branchiae, 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 seiual 

 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 branchiae are at 

 present universally referred to as affording a 

 convenient basis of classification. We shall 

 in this article follow the arrangement adopted 



VOL. II. 



by Ferussac, of which, as well as of the 

 systems of other zoologists, an outline is con- 

 tained in the following table. 



Order I. NUDIBRANCHIATA* (Cuv.) 



Syn. Polykranchiata,^ and genus Derit, 

 Blainville ; Gasteropoda Dermobranches,! Du- 

 meril ; Gasteropoda Tritoniciu, Lamarck. 



In these the branchiae are symmetrical, as- 

 suming a variety of forms, but always placed 

 upon some part of the back, where they are 

 unprotected by any covering; the animals may 

 be provided with a shell or naked, but they 

 are all hermaphrodite with mutual copulation, 

 and marine. 



1st Sub-order, Anthobranchiatafj Goldfuss; 

 Cyclobr<inchititn,\\ Blainville. 



1st Fam. Doris. 

 2d Sub-order, Polybranchiata, Blainville. 



2d Fam. Tritonia, fg. 173. 



3d Fam. Glaucus, fg. 174. 



Fig. 173. 



Fig. 174. 



Order II. INFEROBRANCHIATA, 

 (Cuv. and Blain.) 



Syn. G(itt.l)crmubranches,DumeTil; Cast. 

 Phyllidicni, Lamarck. 



In the Inferobranchiate Gasteropods the 

 branchio: 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 



* Nudui, naked ; branchiae, gilli. 

 t nXuc , many ; branchiae. 

 } Aif,u<, shin. 

 $ AvSoc, a fairer. 

 || KixXoc, a circle. 



