Page:EB1911 - Volume 11.djvu/524

 many cases be useful. Lavage or washing out of the stomach with weak alkaline solutions has been used with marked success in the treatment of chronic gastritis. Of medicinal agents, bismuth, arsenic, nux vomica, and the mineral acids are all of acknowledged efficacy, as are also preparations of pepsin.  GASTROPODA, the second of the five classes of animals constituting the phylum Mollusca. For a discussion of the relationship of the Gastropoda to the remaining classes of the phylum, see.

The Gastropoda are mainly characterized by a loss of symmetry, produced by torsion of the visceral sac. This torsion may be resolved into two successive movements. The first is a ventral flexure in the antero-posterior or sagittal plane; the result of this is to approximate the two ends of the alimentary canal. In development, the openings of the mantle-cavity and the anus are always originally posterior; later they are brought forward ventrally. During this first movement flexure is also produced by the coiling of the visceral sac and shell; primitively the latter was bowl-shaped; but the ventral flexure, which brings together the two extremities of the digestive tube, gives the visceral sac the outline of a more or less acute cone. The shell necessarily takes this form also, and then becomes coiled in a dorsal or anterior plane—that is to say, it becomes exogastric. This condition may be seen in embryonic Patellidae, Fissurellidae and Trochidae (fig. 1, A), and agrees with the method of coiling of a mollusc without lateral torsion, such as Nautilus. But ultimately the coil becomes ventral or endogastric, in consequence of the second torsion movement then apparent. The second movement is a lateral torsion of the visceral mass, the foot remaining a fixed point; this torsion occurs in a plane approximately at right angles to that of the first movement, and carries the pallial aperture and the anus from behind forwards. If, at this moment, the animal were placed with mouth and ventral surface turned towards the observer, this torsion carries the circumanal complex in a clockwise direction (along the right side in dextral forms) through 180° as compared with its primitive condition. The (primitively) right-hand organs of the complex thus become left-hand, and vice versa. The visceral commissure, while still surrounding the digestive tract, becomes looped; its right half, with its proper ganglion, passes to the left side over the dorsal face of the alimentary canal (whence the name supra-intestinal), while the left half passes below towards the right side, thus originating the name infra-intestinal given to this half and to its ganglion. Next, the shell, the coil of which was at first exogastric, being also included in this rotation through 180°, exhibits an endogastric coiling (fig. 1, B, C). This, however, is not generally retained in one plane, and the spire projects, little by little, on the side which was originally left, but finally becomes right (in dextral forms, with a clockwise direction, if viewed from the side of the spire; but counter-clockwise in sinistral forms). Finally, the original symmetry of the circumanal complex vanishes; the anus leaves the centre of the pallial cavity and passes towards the right side (left side in sinistral forms); the organs of this side become atrophied and disappear. The essential feature of the asymmetry of Gastropoda is the atrophy or disappearance of the primitively left half of the circumanal complex (the right half in sinistral forms), including the gill, the auricle, the osphradium, the hypobranchial gland and the kidney.

In dextral Gastropods the only structure found on the topographically right side of the rectum is the genital duct. But this is not part of the primitive complex. It is absent in the most primitive and symmetrical forms, such as Haliotis and Pleurotomaria. Originally the gonads opened into the kidneys. In the most primitive existing Gastropods the gonad opens into the right kidney (Patellidae, Trochidae, Fissurellidae). The gonaduct, therefore, is derived from

the topographically right kidney. The transformation has been actually shown to take place in the development of Paludina. In a dextral Gastropod the shell is coiled in a right-handed spiral from apex to mouth, and the spiral also projects to the right of the median plane of the animal.

When the shell is sinistral the asymmetry of the organs is usually reversed, and there is a complete situs inversus viscerum, the direction of the spiral of the shell corresponding to the position of the organs of the body. Triforis, Physa, Clausilia are examples of sinistral Gastropods, but reversal also occurs as an individual variation among forms normally dextral. But there are forms in which the involution is “hyperstrophic,” that is to say, the turns of the spire projecting but slightly, the spire, after flattening out gradually, finally becomes re-entrant and transformed into a false umbilicus; at the same time that part which corresponds to the umbilicus of forms with a normal coil projects and constitutes a false spire; the coil thus appears to be sinistral, although the asymmetry remains dextral, and the coil of the operculum (always the opposite to that of the shell) sinistral (e.g. Lanistes among Streptoneura, Limacinidae among Opisthobranchia). The same, mutatis mutandis, may occur in sinistral shells.

The problem of the causes of the torsion of the Gastropod body has been much discussed. E. R. Lankester in the ninth edition of this work attributed it to the pressure of the shell and visceral hump towards the right side. He referred also to the nautiloid shell of the larva falling to one side. But these are two distinct processes. In the larva a nautiloid shell is developed which is coiled exogastrically, that is, dorsally, and the pallial cavity is posterior or ventral (fig. 2, C): the larva therefore resembles Nautilus in the relations of body and shell. The shell then rotates towards the left side through 180°, so that it becomes ventral or endogastric (fig. 2, D). The pallial cavity, with its organs, is by this torsion moved up the right side of the larva to the dorsal surface, and thus the left organs become right and vice versa. In the subsequent growth of the shell the spire comes to project on the right side, which was originally the left. Neither the rotation of the shell as a whole nor its helicoid spiral coiling is the immediate cause of the torsion of the body in the individual, for the direction of the torsion is indicated in the segmentation of the ovum, in which there is a complete