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 reversal of the cleavage planes in sinistral as compared with dextral forms. The facts, however, strongly suggest that the original cause of the torsion was the weight of the exogastric shell and visceral hump, which in an animal creeping on its ventral surface necessarily fell over to one side. It is not certain that the projection of the spire to the originally left side of the shell has anything to do with the falling over of the shell to that side. The facts do not support such a suggestion. In the larva there is no projection at the time the torsion takes place. In some forms the coiling disappears in the adult, leaving the shell simply conical as in Patellidae, Fissurellidae, &c., and in some cases the shell is coiled in one plane, e.g. Planorbis. In all these cases the torsion and asymmetry of the body are unaffected.
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| Fig 3.—Sketch of a model designed so as to show the effect oftorsion or rotation of the visceral hump in Streptoneurous Gastropoda. | |
A, Unrotated ancestral condition. B, Quarter-rotation. C, Complete semi-rotation (the limit). an, Anus. ln, rn, Primarily left nephridium and primarily right nephridium. lvg, Primarily left (subsequently the sub-intestinal) visceral ganglion. rvg, Primarily right (subsequently the sub-intestinal) visceral ganglion. | cerg, Cerebral ganglion. plg, Pleural ganglion. pedg, Pedal ganglion. abg, Abdominal ganglion. bucc, Buccal mass. W, Wooden arc representing the base-line of the wall of the visceral hump. x, x′, Pins fastening the elastic cord (representing the visceral nerve loop) to W. |
The characteristic torsion attains its maximum effect among the majority of the Streptoneura. It is followed in some specialized Heteropoda and in the Euthyneura by a torsion in the opposite direction, or detorsion, which brings the anus farther back and untwists the visceral commissure (see Euthyneura, below). This conclusion has shown that the Euthyneura do not represent an archaic form of Gastropoda, but are themselves derived from streptoneurous forms. The difference between the two sub-classes has been shown to be slight; certain of the more archaic Tectibranchia (Actaeon) and Pulmonata (Chilina) still have the visceral commissure long and not untwisted. The fact that all the Euthyneura are hermaphrodite is not a fundamental difference; several Streptoneura are so, likewise Valvata, Oncidiopsis, Marsenina, Odostomia, Bathysciadium, Entoconcha.
Classification.—The class Gastropoda is subdivided as follows:
| Sub-class I. Streptoneura. | |
| Order 1. Aspidobranchia. | |
| Sub-order | 1. Docoglossa. |
| ” | 2. Rhipidoglossa. |
| Order 2. Pectinibranchia. | |
| Sub-order | 1. Taenioglossa. |
| Tribe | 1. Platypoda. |
| ” | 2. Heteropoda. |
| Sub-order | 2. Stenoglossa. |
| Tribe | 1. Rachiglossa. |
| ” | 2. Toxiglossa. |
| Sub-class II. Euthyneura. | |
| Order 1. Opisthobranchia. | |
| Sub-order | 1. Tectibranchia. |
| Tribe | 1. Bullomorpha. |
| ” | 2. Aplysiomorpha. |
| ” | 3. Pleurobranchomorpha. |
| Sub-order | 2. Nudibranchia. |
| Tribe | 1. Tritoniomorpha. |
| ” | 2. Doridomorpha. |
| ” | 3. Eolidomorpha. |
| ” | 4. Elysiomorpha. |
| Order 2. Pulmonata. | |
| Sub-order | 1. Basommatophora. |
| ” | 2. Stylommatophora. |
| Tribe | 1. Holognatha. |
| ” | 2. Agnatha. |
| ” | 3. Elasmognatha. |
| ” | 4. Ditremata. |
Sub-Class I.—Streptoneura
In this division the torsion of the visceral mass and visceral commissure is at its maximum, the latter being twisted into a figure of eight. The right half of the commissure with its ganglion is supra-intestinal, the left half with its ganglion infra-intestinal. In some cases each pleural ganglion is connected with the opposite branch of the visceral commissure by anastomosis with the pallial nerve, a condition which is called dialyneury; or there may be a direct connective from the pleural ganglion to the visceral ganglion of the opposite side, which is called zygoneury. The head bears only one pair of tentacles. The radular teeth are of several different kinds in each transverse row. The heart is usually posterior to the branchia (proso-branchiate). The sexes are usually separate.
The old division into Zygobranchia and Azygobranchia must be abandoned, for the Azygobranchiate Rhipidoglossa have much greater affinity to the Zygobranchiate Haliotidae and Fissurellidae than to the Azygobranchia in general. This is shown by the labial commissure and pedal cords of the nervous system, by the opening of the gonad into the right kidney, and by other points. Further, the Pleurotomariidae have been discovered to possess two branchiae. The sub-class is now divided into two orders: the Aspidobranchia in which the branchia or ctenidium is bipectinate and attached only at its base, and the Pectinibranchia in which the ctenidium is monopectinate and attached to the mantle throughout its length.
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| Fig. 4.—The Common Limpet (Patella vulgata) in its shell, seen fromthe pedal surface. (Lankester.) | |
x, y, The median antero-posterior axis. a, Cephalic tentacle. b, Plantar surface of the foot. c, Free edge of the shell. d, The branchial efferent vessel carrying aerated blood to theauricle, and here interrupting the circlet of gill lamellae. | e, Margin of the mantle-skirt. f, Gill lamellae (not ctenidia, but special pallial growths, comparablewith those of Pleurophyllidia). g, The branchial efferent vessel. h, Factor of the branchial advehent vessel. i, Interspaces between the muscular bundles of the root ofthe foot, causing the separate areae seen in fig. 5, c. |