IV. GASTEROPODA 



327 



holostome shells with entire mouths (C) and siphonostome shells, in 

 which the anterior margin is drawn out in a groove (£) . 



A simple conical shell without further evidence is not proof of primitive 

 structure. It may arise from the spiral form by dej^eneration, 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 connection at the aperture is more intimate, while a muscle is attached to 

 the columella at about its middle, the other end beini; inserted in the foot. This 

 retracts the animal within the shell, the head and anterior part of the foot 



Fig. 330. — Various forms of shells (after Schmarda, Bronn, and Clessin). A 

 Patella costata; B, Haliotis tiibercutata; C,Lilhoglvphus natkoides; D, V crmetus denti- 

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



going first and then the metapodium, with its dorsal surface towards the aper- 

 ture. 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 enlarges in a spiral manner (fig. 330, C), some 

 times forming a spiral line on the outside, or on both surfaces. Land snails 

 are usually without opercula, but in hibernation they can close the shell by a 

 calcareous plate, the epiphragm. 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. 

 331) the whorls are constantly turned to the left, while reversed specimens 

 occasionally occur in many species which are normally dextral. 



There are usually two layers in the shell, an inner lamellar layer (not always 

 present), which sometimes is highly iridescent, and an outer opaque, porcellan- 



