74 



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 Molluscs (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 ; li, a stage 1 hour later than A \ C, a stagf 3i 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 



A 



FIG. 52. 



Diagram of the torsion of the visceral commissure in the Streptonenra seen from the buccal 

 side. A, before the torsion, whose direction is indicated by the arrow ; B, after the torsion, 

 a, anus ; a&, abdominal ganglion ; ce, cerebral ganglion ; i.i, infra-intestinal ganglion ; in, 

 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, 



