Its opening, the vent, is situated either on the middle line at the base of the tail, or on the right side, as if to balance the sinistral position of the spiraculum.
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.
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.
The same, mutatis mutandis, may occur in sinistral shells.
Burr.- p T 9 pl.y ped.g: 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.
Lanistes, shell sinistral, spire short or obsolete.
With sinistral spiral; pallial cavity dorsal.
In Planorbis, which is sinistral (as are a few other genera or exceptional varieties of various Anisopleurous Gastropods).
Visceral mass and shell sinistral; inferior pallial lobe very prominent, and transformed into a branchia.
Shell bulimoid, dextral or sinistral; radular teeth, expanded at their extremities and multicuspidate.
The shell of Clausilia is sinistral and its aperture is provided with a hinged plate.
It projects horizontally forwards from the head in the form of a cylindrical or slightly tapering, pointed tusk, composed of ivory, with a central cavity reaching almost to the apex, without enamel, and with the surface marked by spiral grooves and ridges, running in a sinistral direction.
Lanceolatus obtained by the tow-net in 8 -10 fathoms, showing the asymmetry of the large lateral sinistral mouth with its ciliated margin cm and the dextral series of simple primary gill-slits (Ips-14ps).