74 THE GASTROPODA 
in the Cavoliniidae and Cymbuliidae, there is a space left between 
the mantle and the body, constituting the pallial cavity. This 
pallial cavity has a situation opposite to that which it occupies in 
other Molluses (Fig. 22), due to the torsion which the Gastropod 
body undergoes towards the end of its development. 
Fig. 51. 
Three larvae (veligers) of Trochus, during the process of torsion, viewed from the right side 
of the shell. A, nearly symmetrical larva; B, astage 14 hour later than 4 ; C, a stage 3} hours 
later than B. f, foot; op, operculum ; pa.c, pallial cavity ; ve, velum. (After Robert.) 
The process by which this torsion is brought about may be 
referred, on ultimate analysis, to a morphological phenomenon 
common to the Cephalopods (Fig. 119, D), Scaphopods (Fig. 119, B), 
and Lamellibranchs (Fig. 119, C), as well as to the Gastropods. 
This phenomenon is the ventral flexure which takes place in an 
antero-posterior sagittal plane, about a transverse axis situated at 
Diagram of the torsion of the visceral commissure in the Streptoneura seen from the buccal 
side. A, before the torsion, whose direction is indicated by the arrow; B, after the torsion. 
a, anus; ab, abdominal ganglion; ce, cerebral ganglion; 7.7, infra-intestinal ganglion; m, 
mouth ; pa.c, pallial cavity ; s.i, supra-intestinal ganglion. 
right angles to the main antero-posterior axis of the animal, and results 
in the approximation of the two ends of the digestive canal. As 
a consequence of this flexure, the visceral mass and shell, which were 
originally saucer-shaped, become thimble-shaped, or are produced 
into a more or less pointed cone, and during the flexure there is a 
simultaneous coiling of the visceral sac and the shell covering it in 
a dorsal or anterior direction, so that an exogastric coil is produced, 
