THE GASTROPODA 



77 



has been confirmed by the study of the embryology of Paludina. 

 (4) The coil of the visceral sac and shell becomes endogastric. 

 Originally these structures were coiled dorsally or, in other words, 

 were exogastric (Fig. 53, C), but as a result of a rotation through 

 an angle of 180, the coil necessarily becomes ventral or endo- 

 gastric (Fig. 53, D). Most usually, however, the coils of the 

 visceral sac and shell do not remain in the same plane, but the 

 summit of the spire gradually comes to project on the side which 

 was originally left, but which at the end of development is finally 

 and topographically right (Fig. 44). Thus a spiral coil is formed 

 which has the advantage of giving a more compact form to the 

 shell and its contents, and of diminishing its diameter. In those 

 forms in which the torsion and asymmetry is dextral, the coil of 

 the spire is conformable since it also is dextral ; that is to say, it 

 follows the direction of the hands of a watch if the shell is viewed 

 from the summit of the spire (Figs. 47 and 132, etc.). Nevertheless, 

 the coil of the shell is by no 

 means the cause of the torsion ; 

 both are foreshadowed in the 

 segmentation of the ovum, in 

 which there is a complete reversal 

 of the direction of the cleavage 

 planes in sinistral as compared 

 with dextral Gastropods. The 

 apparent direction of the coil, 

 however, may be changed by a pro- 

 cess of liyperstrophy (see below, 

 p. 82), and finally the coil of the 



Fio. 57. 



Nervous system of Actaeon torna- 

 tilis, in situ, dorsal aspect. I, buccal 

 gland ; II, buccal mass ; III, cerebral 

 ganglion; IV, infra -intestinal part 

 of the visceral commissure, with a 

 small pallial ganglion ; V, infra- 

 intestinal ganglion ; VI, right sali- 

 vary gland ; VII, abdominal ganglion 

 and genital nerve ; VIII, oesophagus ; 

 IX, supra - intestinal part of the 

 visceral commissure ; . X, supra- 

 intestinal ganglion. 



FIG. 50. 



Shell of a very young Patella 

 vulgata, viewed from the right 

 side, x 25. sp, apical spire. 



visceral mass and spire may disappear in the aduL, ieaving the in- 

 ternal torsion and asymmetry unaltered, but producing a secondary 

 external symmetry, as in the Patellidae (Fig. 56), Fissurellidae, etc. 

 (5) By de torsion, or movement in a contrary direction, the anus 

 and circumanal complex (with the exception of the male or herma- 

 phrodite genital aperture) may be carried back to a posterior posi- 



