64 OUTLINES OF XATURAL HISTORY. 



wards ; and the animal has the power of shifting its posi- 

 tion within its burrow by protruding its "foot" — this, as 

 already said, being a tongue-shaped muscular organ (fig. 

 24, /j, which can be thrust out between the valves of the 

 shell. The length of the shell may be over three or four 

 inches, with a breadth of about two inches and a half. 



Recapitulation of Essential Characters. — The 

 body is soft, and is enclosed in a skin or integument 

 which is termed the "mantle." The mantle produces a 

 " shell," which protects the soft body within, and which 

 consists of two pieces or "valves," placed one upon the 

 right side and one upon the left side of the body. There 

 is no distinct head, and the mouth is destitute of teeth. 

 The breathing-organs are in the form of lamellar or plate- 

 like gills, disposed on the sides of the body. These 

 characters distinguish the class of the Lamellibranchiata 

 as a whole. 



CHAPTER XVII. 



class gasteropoda. 



In this class are included all those animals which are com- 

 monly called " Univalve Shell-fish," such as the Whelks, 

 Snails, Periwinkles, Limpets, &c. The name of "Uni- 

 valves " is applied to them because most of them possess 

 a shell which is composed of a single piece or "valve;" 

 and they derive the name of Gasteropoda from the fact 

 that the lower surface of the body is generally flattened 

 out so as to form a broad expansion or disc, which is 

 called the "foot," and ujjon which the animal creeps 

 about (Greek, gaster, belly ; podes, feet). As the type of 

 this class we may select the common "N^Tielk {Buccinum 

 undatum) of British seas. 



If we examine a "Whelk whilst living and active, we 



