328 



MOLLUSCA 



ous layer, contains the pigment and may be covered by a horny cuticle. In rare 

 cases mantle and shell are lacking, or the mantle is present but the shell is rudi- 

 mentary and not visible externally because the mantle folds have grown over it. 

 In these cases the visceral sac is not prominent. Since the shellless forms 

 possess a mantle and shell in the young, the adult conditions are explained by 

 degeneration. 



Only a few gasteropods are bilaterally symmetrical. Usually the spiral 

 twist of the visceral sac has resulted in a torsion of other parts from left to 



right, in which alimentary tract, nephridia, gills, 

 heart, and nervous system take part (fig. 332). The 

 intestine is bent in this way, the anus opening into 

 the mantle chamber on the right side (B) or the 

 twisting may be continued so far as to double the 

 intestine on itself, the anus being in the middle line 

 in front, near the head (C). Nephridia, gills (with 

 them the osphradia), and heart wander in company, 

 ^^^^__ so that the organs primitively belonging on the left 



side may be transferred to the right and vice versa. With this there is 

 a tendency to asymmetry and the loss of the organs (usually of the 

 primitively left side). When the nervous system takes part in the twist- 



FIG. 331. Sinis- 

 tral shell of Lanistex 

 carinatus (from 

 Leunis). 



FIG. 332. Three diagrams illustrating the torsion of the body and the twisting of 

 the nervous system in gasteropods (after Lang). A, bilateral; B, asymmetrical; 

 C, streptoneurous condition. The reference letters are placed upon the organs of the 

 primitive left side, a, anus; C, cerebral ganglion; g, ctenidium; /, auricle; m, mouth; 

 , nephridial opening; o, osphradium; pa, parietal ganglion; pc, pedal ganglion; pi, 

 pleural ganglion; v, ventricle. 



ing a crossing of the cerebrovisceral commissures takes place, known as 

 streptoneury or cliiastoneitry (C). 



The alimentary canal begins with a muscular region which in some 

 groups is developed into a large protrusible proboscis (fig. 333). The 



