114 A Guide to the Zoological Collections in the 



the close produced spiral is shown in Turritella^ Case 

 140X, and the close plane spiral in Rotella, Case 131 A, and 

 Marisa 148 A. All manner of intermediate forms occur. 



The mouth of the shell, which is polished and often 

 thickened and everted, is known as the '' peritreme", and 

 the axis round which the shell is coiled is the "columella", 

 each turn of the coil being called a " whorl ". The 

 columella is sometimes closed below by the peritreme, 

 but sometimes remains open, the opening being known as 

 the " umbilicus '\ 



The mouth of the shell can commonly be closed by a 

 horny or shelly plate known as the '' operculum ": this is 

 secreted by the upper surface of the hinder portion of the 

 foot, or metapodium. 



There are two subdivisions of this very large Class— 

 (i) the Gastropoda Jsopleura, in which the body with all 

 its contents is bilaterally symmetrical ; and (2) the Gastro- 

 poda Atiisopleura, in which only the head and foot are 

 bilaterally symmetrical, while the visceral mass and 

 mantle are twisted completely over to the right side, the 

 intestine ending alongside or above the animal's head. 



a. GASTROPODA ISOPLEURA. 



[Cases I26jl-X26f ]. 



The principal Order of this sub-Class is that of the 

 Polyplacophora or Chitons. 



The Chitons are distinguished among all Gastropoda 

 by the nature of the shell, which consists of eight imbricat- 

 ing plates arranged in a row, and deeply immersed in the 

 rough leathery mantle. They are common inhabitants of 

 the rocks of all shores, and are well represented in Case 

 126A-B. The sexes in the Chitons are distinct. 



b. GASTROPODA ANISOPLEURA. 



The anisopleurous or asymmetrical Gastropods are 

 subdivided into four Orders according to the nature of 

 the breathing organs, namely, (i) the Zygohranchia 

 with a pair of gill-plumes ; (2) the Azygobranchia with 



