20 



LAND AND FRESH-WATER SHELLS. 



Sometimes a shell becomes shaped like a spiral ladder, or 

 scalarid, as in the accompanying figure of the m. scalariforme of 

 our common Wrinkled Snail. The shell in Arion may be absent, 



Fig. 4. Helix aspersu. t ... ^calarifortnt. 



or may consist of a few calcareous granules: in Limax, the shell 

 is shield-like ; in both it is situated underneath the mantle. The 

 grades may be traced from these simple shells to the more 

 developed shells of the Helices, through the little ear-shaped 

 shell on the tail of Testacella haltotidea, and the pretty glassy- 

 green shell of Vitrina pdlucida. 



THE BODY-WALL CONSIDERED GENERALLY. In the Limacldcz 

 the mantle is visible throughout its whole extent ; in Limnx the 

 respiratory orifice is on its posterior half; in Arion and Geoma- 

 lacus on its anterior half. It may partly overlap the shell, as in 



Fig. 5. A slug (Lima*) showing ihc respiratory orifice. 



Amphipeplea glutinosa and in Physa fvntinalis, or its fore-edge may 

 undergo concrescence with the dorsum of the body-wall, as in the 

 Heliridce. In the Limnace. the fore-edge is free, and no such con- 

 crescence has taken place. In Valvata the long plume-like gills 

 protrude from the mantle cavity when the animal is walking. 



There may be four cylindrical tentacles, as in the Limaddce and 

 Hdicidcz, or only two, as in Limr,cea and in Physa. When two 



