JV. GASTEROPODA. 



371 



it, causing its color pattern. When the siphon is present the 

 shell shows a corresponding process. Thus are distinguished 

 holostomate shells with smooth mouths (fig. 302, I>) and sfphono- 



FiG. :j62.— Various forms of sheUs. (After Schmarda, Bronn, and Clessin.) A, Patella 

 eiixtata ; B, Haliotis tuberculata ; C, Vci-metus (Jentiferut: ; D, Lithmjlyphus naticoidea ; 

 E, sheU of Murex opened to show c, columeUa ; o, siphon. 



stome shells, in which the anterior margin is drawn out in a 

 groove (fig. 302, E). 



A simple conical shell without further evidence is not proof of primi- 

 tive structure. It may arise fi-om the spiral form by degeneration, if the 

 visceral sac be reduced. Thus the shells of FissnreUa und Patella are to be 

 e-xplained, for the viscera here show the results of an earlier spiral twist. 



In most places the union between shell and soft parts is not very firm, 

 but the conne.xion at the aperture is more intimate, while a muscle is at- 

 tached to the columella (musoulus columellaris) at about the middle point 

 of its height, the other end being in.serted in the foot. It is for the retrac- 

 tion of the animal within the shell, first the anterior part with the head 

 and then the rest witli the metapodium. In this the metapodium is folded 

 so that its dorsal surface lies towards the aperture. Itence in many species 

 this surface secretes a door, or operciiluin, which closes the aperture when 

 the body retracts. Since the aperture increases in size with growth, the 

 operculum must also enlarge, whicli is accomplished in a spiral manner 

 (flg. 362, D), the process sometimes showing in a s]iiral line on the out- 

 side. So-called eye stones are the opercula of small Trochida^ and Tur- 

 binida;. Land snails are usually without opercula, but at certain times, 



