372 



MOLLUSC A. 



as in hibernation, they can close the shell by a calcareous plate, the epi- 

 pJiragm. In the spring this separates from the shell and is lost. 



In most gasteropods the shell is coiled to the right, but in some species 

 (fig. 363) the whorls are constantly turned to the left, 

 while reversed specimens occasionally occur in many 

 species which are normally dextral. 



In the shell there are at most two layers, an inner 

 lamellar layer (not always present), which sometimes 

 is highly iridescent, and an outer porcellanous layer, 

 which is P a( l ue and contains the pigment. In rare 

 of Lanistes carinatus. cases the mantle and consequently the shell are lack- 

 sT n u 'ing, 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 shell- 

 less forms possess a mantle and shell in the young, the adult conditions 

 are explained by degeneration. 



Only a few gasteropods are like the Amphineura and Acephala 

 in being 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 nerv- 

 ous system take part. The intestine is bent in this way, the anus 

 opening into the mantle chamber on the right side, 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. Nephridia, 



TTlG. 364. 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: 0, ctenidium; 7, auri- 

 cle; w, mouth; n, nephridial opening; o, osphradium; pa, parietal ganglion; 

 pe, pedal ganglion; pi, pleural ganglion; v, ventricle. 



gills (with them the osphradia), and heart wander in company, so 

 that the organs primitively belonging on the left side may be trans- 



