IV. 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. 362, D) and siphono- 



FIG. 362. Various forms of shells. (After Schmarda, Bronn, and Clessin.) A, Patella 

 costata: B, Haliotis tuberculata; C. Vermetus dentiferm ; D, Lithoglyphus naticoides; 

 E, shell of Murex opened to show c, columella ; 8, siphon. 



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

 groove (fig. 362, E). 



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

 tive structure. It may arise from the spiral form by degeneration, if the 

 visceral sac be reduced. Thus the shells of Fissurella and Patella are to be 

 explained, 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 connexion at the aperture is more intimate, while a muscle is at- 

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

 of its height, the other end being inserted 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 with the metapodiuin. In this the metapodium is folded 

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

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

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

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

 (fig. 362, D), the process sometimes showing in a spiral line on the out- 

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

 binida3. Land snails are usually without opercula, but at certain times, 



