248 SUBKINGDOM MOLLUSCA. 



Helicidae (spiral-form). The Land-snail has four tenta- 

 cles, the longer pair with eyes at the extremity. It lays its 

 eggs singly, while the other Orders lay theirs in chains.* 



Limacidae. The Slugs are snails without the shell, or 

 with only a rudimentary one concealed in the mantle. 



ORDER OPISTHOBRANCHIATA. 



General Characteristics. The Opisthobranchs (gills 

 behind) are generally naked, with feathery gills behind the 

 heart. 



Jffife, Britain. 



The Soli's is typical of the order, having its gills arranged 

 in tufts along the back, and the tentacles non-retractile. 



ORDER NUCLEOBRANCHIATA. 

 General Characteristics. The Nucleobranchs (clus- 



Mg. W7. 



Carinatia mediterranZa. 



* If placed upon a pane of ejlass, by looking underneath, one can easily observe 

 Its curious progression by means of a single foot. 



