MOLLTJSCA : GASTEEOPODA. 



263 



The Holostomata, of which the Common Periwinkle (Litto- 

 rina littorea) is a good example, are either spiral, or limpet- 



Fig. 79. Gasteropoda, a. Holostomatous shell (Turntellacommunis). b. Sipho- 

 nostomatous shell (Buccinum undalum). 



shaped, in some few instances tubular, or multivalve ; the aper- 

 ture of the shell being in most cases entire. They are mostly 

 plant-eaters, and they may be either marine or inhabitants of 

 fresh water. The following families are included in this sec- 

 tion : Natiddo&i Pyramidellidce, Cerithiadce, Melaniadce, Turri- 

 tellidaR, Littorinida} (Periwinkles), Paludinidce (River-snails), 

 Neritidce, Turbinidce (Top-shells), Haliotidm (Ear-shells), Fis- 

 surellidce (Key-hole Limpets), Calyptrceidce (Bonnet Limpets), 

 PatellidoR (Limpets), Dentalidoe (Tooth-shells), and Chitonidcv. 



ORDEE II. OPISTHOBEANCHIATA. This order is denned as 

 follows : 



' ft hell rudimentary, or wanting. Branchiae arborescent or 

 fasciculated, not contained in a special cavity, but more or less 

 completely exposed on the back and sides, towards the rear 

 (opisthen) of the body. Sexes united.' (M. -Edwards. See 

 Woodward's Manual.) 



The Opistkobranchiata, or * Sea-slugs,' may be divided into 

 two sections, the Tectibranchiata and Nudibranchiafa, accord- 

 ing as the branchiae are protected or are uncovered. 



The first section, that of the Tectibranchiata, is distinguished 

 by the fact that the animal is usually provided with a 

 shell, both in the larval and adult state, and that the 

 branchiae are protected by the shell, or by the mantle. Under 

 this family are included the families of the Tornatellidce, Bui- 



