THE GASTROPODA fe 
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, ete.). 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 hyperstrophy (see below, 
p- 82), and finally the coil of the 
Fie. 57. 
Nervous system of Actaeon torna- 
tilis, in situ, dorsal aspect. I, buecal 
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- 
Fic. 56. vary gland ; VII, abdominal ‘ganglion 
and genital nerve ; VIII, oesophagus ; 
Shell of a very young Patella IX, supra - intestinal ‘part of the 
vulgata, viewed from the right visceral commissure; X, supra- 
side, x 25. sp, apical spire. intestinal ganglion. 
visceral mass and spire may disappear in the adult, leaving the in- 
ternal torsion and asymmetry unaltered, but producing a secondary 
external symmetry, as in the Patellidae (Fig. 56), Fissurellidae, ete. 
(5) By detorsion, 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- 
