206 GASTEROPODS. 



ORDER PULMONIFERA.* 



The Air-breathing Gasteropods, of which the slug and the 

 snail are familiar examples, respire atmospheric air, which is 

 alternately drawn into and expelled from a cavity lined with a 

 most delicate network of blood-vessels : this respiratory organ 

 opens externally on the right side of the body, near the margin 

 of the shell, below the collar of the mantle. Some are terrestrial, 

 others live in streams or in sluggish stagnant waters; some are 

 shelled, others are naked. 



The Terrestrial Air-breathing Gasteropods are at once 

 recognizable by their four tentacula, or horns, as they are com- 

 monly called. These appendages are retractile, and the upper pair 

 have eyes at their extremities. The mouth is armed with a broad 

 cutting tooth, and a wonderfully-constructed tongue, studded with 

 innumerable microscopic teeth. Some are only provided with 

 an internal shell, while others have an external shell spirally 

 twisted. 



FlG. 211. SXAILS AND SLUGS. 



The Slugs (Lima.v) are without any apparent shell ; their mantle is a 

 fleshy disk scarcely separated from the rest of the skin, and only occupies the 



* Pulmo, a lung; fero, to carry. 



