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 Molluscs 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 pallia! 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- 

 volving all the organs contained 

 in the shell the cephalo- pedal 



Fid. 53. 



Four stages of the development of a Gastropod, 

 showing the process of the body -torsion. A, 

 embryo without flexure ; />, embryo with ventral 

 flexure of the intestine ; C, embryo with ventral 

 flexure and an exogastric shell ; D, embryo with 

 lateral torsion and an endogastric shell (the 

 arrows indicate the direction of the torsion), 

 a, anus ; /, foot ; m, mouth ; pa, mantle ; pa.c, 

 }>allial cavity ; ve, velum. (After Robert.) 



Fio. 54. 



Scisturdla lytteltonensis, but of its 

 shell, dorsal aspect I, snotit ; II, right 

 tentacle; III, pallial slit; IV, right 

 gill; V, rectum; VI, gonad ; VII, left 

 kidney ; VIII, left half of the columellar 

 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 



