MOLLUSC A. 



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

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



In most gnstei'opods the shell is coiled to the right, but in some species 

 (fig. 3G3) 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, 

 FiG.3tB.-sinistralshell""l"* is opaque and contains the pigment. In rare 

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



(From Ludwig-Leu- . , , . » u ,. tu i 11 • t 



nis) 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 oases 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 Ampliineura and Acepliala 

 in being bilaterally symmetrical. Usually the sjiiral t^vist of the 

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

 right, in Avhich 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. Xephridia, 



Fig. 3&t.— Three diaErams illustratint.' the torsion of the bodv and the twlstins of the 

 nervous system in gaeteropods. (.\ftcr Lang.) .1, bilateral, II, asvnimetrical, 

 G, streptoneurous condition. Tlie rufereiue letters are plaeed upon' the orsrans 

 of the primitive left side. «, anus; r, cerebral ganglion ;(/, ctenidiuni: /.auri- 

 cle; m, mouth; 11, nephridia.l opening; .), ospliradium; ^ju, parietal ganglion; 

 pe, pedal ganglion; ()/, pleural ganglion; r, ventricle. 



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

 that tlic organs ])riiiiitively 1)el<uiging on the left side may be trans- 



