VII MOLLUSC A— THE ASYMMETRY OF THE GASTROPODA 155 



development of the shell during which its displacement in a backward direction and the 

 shifting forward of the pallial complex occur. When once the result most favourable 

 to the animal, i.e. the anterior position of the mantle cavity and the backwai'd 

 direction of the shell, are attained, further displacement, which would be dis- 

 advantageous, does not take place. It is, then, not at first sight evident why, 

 when the need for displacement ceases, its action still continues, i.e. why, though 

 displacement ceases, the visceral dome and shell continue to grow in a dextral 

 twist and not symmetrically. This point will be explained below. 



For the sake of clearness we liave treated separately the three important factors 

 in the development of the Gastropod shell, viz. (1) the formation of a tall conical 

 shell, (2) the spiral coiling of the same, and (3) the special manner of coiling in a 

 dextral twist. In reality these three factors do not denote special stages, but all 

 operate simultaneously. The continually increasing protrusion of the visceral dome 

 was accompanied by the dextral twist, as a consequence of the twisting of the 

 visceral dome from its incline to the left to the most favourable backward incline, 

 by which the pallial complex was shifted forward. 



The results of ontogenetic research favour the theory here advanced. We 

 have first to note the fact that the anus (the centre of the pallial complex) and the 

 mantle fold originally lie posteriorly. They come to lie anteriorly in the embryo 

 not by active shifting, but by the cessation of growth on the right side between the 

 mouth and anns, and its continuation on tiie left side. There is, however, no 

 difficulty in harmonising this ontogenetic method of gaining the object with the 

 phylogenetic method. 



9. 



So far we have placed mechanical and geometrical considerations in the fore- 

 ground. But these necessarily coincide with utilitarian considerations. Every 

 alteration in the direction we have been considering means an improvement in the 

 organisation of the animal, an advantage to enable it the better to maintain the 

 struggle for existence. The formation of a spire-like shell, which has been recog- 

 nised as the starting-point in the development of the asymmetry of reptant Gastro- 

 pods, was the only method by which complete protection of the whole body could 

 be attained, and must therefore be considered to have been advantageous under the 

 circumstances. We might further conclude this from the fact that the jjossession 

 of such a shell actually distinguishes the Gastropoda from the primitive MoUusca, 

 which the Chitonidcc are rightly considered most nearly to represent. 



10. 



One apparently important objection to the theory here set forth must be mentioned. 

 If the first factor in the asymmetiy of the Gastropod body is the development of a 

 high spire-like shell, and if the arrangement of thenervous system is necessarily 

 connected with the coiling of the shell in a definite direction, how can we account 

 for forms such as Fissurella ? This Diotocardian genus actually belongs to the most 

 primitive Gastropods, because the symmetry of the pallial complex is still retained. 

 But it possesses an asymmetrical nervous system and the typical chiastoneury of 

 the Prosobranchia, and nevertheless a flat cup-shaped symmetrical shell. We thus 

 here have secondary characteristics of the inner organisation combined with an 



