384 



MOLLUSCA. 



exogastric (Fig. 303). But when the visceral sac has undergone 

 the torsion just described, which resulted in its anterior face becoming 

 posterior, this coil would naturally be on the hinder face of the 

 visceral sac (Fig. 303). Such a coil is secondarily endogastric and 

 is the permanent condition in the ordinary dextral visceral sac of 

 most Gastropoda. 



.v.s 



l.v. 



l.ct 



ret. 



I aur 



let 



I au,r 



let 



U 



f 



FIG. 302. Diagrams illustrating the torsion of the visceral sac, which is supposed to have 

 taken place in most Gastropoda (after Lang). A represents the hypothetical primitive form in 

 which no torsion has taken place. In B the visceral sac has rotated through an angle of 45 

 in a direction opposite to that of the hands of a watch ; the mantle-cavity with the gills and 

 anus, etc. is now placed on the right side of the body. C, Intermediate stage. D, final 

 stage, in which the visceral sac has rotated through an angle of 180 in a direction opposite 

 to that of the hands of a watch, and about a dorso- ventral axis ; the mantle-cavity with 

 gills, anus, etc. are now on the anterior side of the visceral sac, and the primitive right gills, 

 kidney, auricle, have become placed on the left side. The visceral commissure has become 

 twisted into a figure of 8 loop. The outline of the visceral sac is indicated by the dotted 

 line, a anus ; l.aur left auricle ; in C, l.aur points to the right auricle ; / foot ; l.ct primitive 

 left gill ; l.v primitive left visceral ganglion (sub-intestinal); m mouth ; man edge of mantle- 

 fold ; r.ct primitive right gill ; r.v primitive right visceral ganglion (supra-intestinal) ; 

 t tentacle ; v ventricle ; v.c visceral commissure ; v.s outline of visceral sac. 



