206 



THE SCIENCE OF LIFE. 



the simplest form of lung a cavity lined with a delicate 

 net-work of blood-vessels, which opens externally on the 

 right side of the neck. This opening is covered by a 

 valve. They are found in all zones, but most where lime 

 and moisture abound. All feed on vegetable matter. A 

 few are naked, as the Slug ; some are terrestrial ; oth- 

 ers live in fresh water. The Land-snails, as the Helix, 

 Bulimus, and Limax, (Slug,) have four horns, the short 

 front pair being the true tentacles, and the long hinder 

 pair the telescopic eyes. The Pond-snails, as Limnaa 

 and Planorbis, have no eye-stalks, the eyes being at the 

 base of the tentacles. They are obliged to come to the 

 surface of the water to breathe. (Fig. 105.) 



FIG. 106. Chiton. 



FIG. 107. Fissurella Reticulata. 



4. Prosobranclis. 

 (Gr.^proson, before 



Having gills in front of the heart. 

 bragchia, a gill.) These are aquatic 

 and generally marine animals, the 

 most highly organized and most 

 abundant of all the Gasteropods. 



Among the lower forms ate the 

 singular Chiton, (Fig. 106,) covered 

 with eight shelly plates ; Limpet, (Pa- 

 tella^] well known to every sea-side 

 visitor ; and the beautiful Ear-shell Abalone, (Haliotis^) 

 (Fig. 108,) often used for ornamental work and jewelry. 



FIG. 108. Ear-shell, or 

 Haliotis. Reduced. 



