154 COMPARATIVE ANATOMY chap. 



parts of the body not covered by the shell there would come a time when the body 

 could no longer be completely withdrawn into it. 



Before discussing the third stage in the development of the Gastropod shell, we 

 must consider its growth. This, from a geometrical point of view, is of three kinds : 

 growth in height, peripheral growth, and radial growth or increased thickness of 

 the shell wall. This last does not here concern us. 



Supposing, for simplicity's sake, the shell to be conical, growth in height occurs 

 at the base (or aperture of the shell), and takes place by means of continual deposits 

 of bands of new material at the edge of the aperture, by the growing edge of the 

 mantle. 



Peripheral growth is the enlargement of the circumference of the base or aperture 

 of the shell. 



If the height and the peripheral growth remain uniform round the whole 

 aperture of the cone (which is assumed to be round), -the cone increases without 

 altering its shape. 



If, however, the growth in height is not uniform, but steadily and symmetrically 

 increases along each side from an imaginary minimum point to a diametrically 

 opposite maximum point, the peripheral growth, however, remaining uniform, a 

 spirally twisted hollow cone is produced. 



If the minimum and maximum points in this growth continue throughout in 

 one and the same plane, a symmetrical shell coiled in this plane of symmetry 

 results. 



If, however, as growth increases, the maximum point shifts from the symmetrical 

 plane, say to the left (the minimum point shifting in the opposite direction to the 

 right), the maximum and minimum points no longer trace on the spirally coiled 

 shell straight but spirally twisted lines, and the conical shell is then not coiled 

 symmetrically in one plane, but asymmetrically in a screw-like spiral. We then 

 have what conchologists call a dextrally twisted shell. 



The growth of the Gastropod shell actually takes place in this last manner. 



This, the dextral (or sinistral) coiling of the Gastropod shell, is the last stage to be 

 discussed. If the visceral dome and shell which are twisted in one plane pass, in growth, 

 from an incline to the left to a backward incline, this is equivalent to the continual 

 shifting of the point of maximum growth to the left and that of minimum growth 

 to the right ; the necessary consequence being a dextral screw-like spiral twist. 



It must be borne in mind — 



1. That the peripheral growth remains constant, i.e. that the outline of the 

 growing edge of the mantle remaining uniform, the increasing aperture of the shell 

 also retains the same form. 



2. That the additions to the shell by the mantle edge are made in the form of 

 bands of new material, the already formed firm shell not altering in shape. 



3. That the growing edge of the mantle, which secretes the shell substance, does 

 not, in the course of the gradual change from the left to the backward incline, itself 

 become twisted, but retains its position in relation to the rest of the body. It is 

 thus only the maximum and minimum points of growth in height which become 

 shifted along the edge of the mantle. 



i. It must be noted that this description of the manner in which a dextrally 

 twisted shell arose only applies to that stage in the ontogenetic or phylogenetic 



