THE GASTROPODA 75 
as may be seen in the Patellidae, Fissurellidae, and Trochidae 
(Fig. 51, A). This disposition of the shell is the same as that which 
obtains in other Molluses with coiled shells (Nautilidae, Fig. 270), 
but without lateral torsion. But in Gastropods, during the com- 
pletion of the metamorphosis, there is a lateral torsion subsequent 
to the primitive ventral flexure, as a result of which the originally 
dorsal or exogastric shell becomes ventral or endogastric (Fig. 51, C). 
This lateral torsion is causally connected with the growth of the 
ventral creeping surface, which primitively was very short, but 
eventually increases in length, and in so doing tends again to 
remove the pallial opening, and with it the anal and renal orifices 
and the respiratory organs, away from the head. The approxima- 
tion of these organs to the head is therefore necessarily effected by 
a lateral torsion in a plane perpendicular to the primitive ventral 
flexure ; that is to say, about a dorso-ventral axis situated in the 
same median sagittal plane as the antero-posterior axis. It is this 
second lateral torsion, then, in- 
S we S volving all the organs contained 
peal in the shell—the cephalo-pedal 
a 
£ ty : 
ei : t 
Four stages of the development of a Gastropod, 
showing the process of the body-torsion. 4, Fic. 54. 
embryo without flexure ; B, embryo with ventral 
flexure of the intestine ; C, embryo with ventral Scissurella lytteltonensis, out of its 
flexure and an exogastric shell; D, embryo with shell, dorsal aspect. I, snout; II, right 
lateral torsion and an endogastric shell (the tentacle; III, pallial slit; IV, right 
arrows indicate the direction of the torsion). gill; V, rectum; VI, gonad; VII, left 
a, anus; f, foot; m, mouth; pa, mantle; pa.c, kidney ; VIII, left half of the columellar 
pallial cavity ; ve, velum. (After Robert.) muscle ; IX, left gill; X, left eye. 
mass being supposed to be fixed or vice versa—which brings the 
pallial aperture and the anus from a posterior to an anterior position 
(Fig. 53). 
During this lateral torsion the following changes are necessarily 
produced in the original organisation of Gastropods:—(1) The 
anus being carried forward along one side of the animal, the 
organs situated on either side of this orifice change their relative 
positions ; those which were morphologically on the right become 
