VII MOLLUSCA~THE ASYMMETRY OF THE GASTROPODA 151 



covering protrude liernia-like in the form of a high spire-like visceral dome, with 

 •which the shell corresponds in shape. The uncoiled shell of every snail is as a 

 matter of fact sjjire-shaped. 



The develoj)raent of such a shell and dome has already been recognised as due to 

 the increased protection needed by the body when the capacity for creeping becomes 

 developed. The whole of the softer part of the body can be withdrawn into such a 

 shell, and, further to increase the protection, an operculum is often developed on the 

 foot for closing the aperture of the shell, when the animal has retired into it. The 

 shell muscle of the racial form no longer serves for pressing the shell against the 

 surface on which it rests, but for withdrawing the head and foot into the shell. It 

 becomes the columellar muscle (Fig. 131, sm). 



Taking in turn the different stages in the development of the Gastropod 



Fig. 131. 



(Lettering in this and in the following three 



figures the same as in Fig. 129.) 



Fig. 130.— Hypothetical primitive Gastropod, 



from above, o, Mouth ; idc, vlp!, ulp, original 

 left cerebral, pleural and pedal ganglia ; tdpa, 

 vrpa, original left and right parietal ganglia ; 

 ^l.la, original left auricles ; uos, uros, original left 

 and right osphradia (Spengel's organs) ; ulct, urct, 

 original left and right ctenidia (gills) ; mb, base 

 of the mantle ; mr, edge of the mantle ; vi, mantle 

 cavity; v, visceral ganglion; vc, ventricle; a, 

 anus. 



shell, we have as the first and most important its dorsal spire-like prolongation. 

 In this way the cup-shaped shell of the racial form becomes a high conical shell like 

 that of Dental ium. 



Such a shell carried vertically liy tlie animal (Fig. 1.31) would, when the latter is 

 at rest, be in a state of unstable equilibrium, which would be upset by movement or 

 by the slightest pressure from without. It is also evident that when the animal is 

 in motion a vertically placed spire-like shell would be extremely awkward. 



If we assume the shell to be carried at some other angle to the body, we have 

 the following possible positions : — 



1. The .shell might be carried inclined forward (Fig. 132). Such a position is 

 the most unfavoiirable imaginable for locomotion, for the functions of the mouth, 

 and for the sensory organs on the head. 



