CHAPTER XI 



THE SLUG AND ITS ALLIES 



THE slug belongs to the group of Mollusca. 1 This 

 group contains animals which possess two distinctive 

 organs the foot^ by means of which locomotion is 

 effected, and the mantle, a fold of skin covering over 

 or enclosing a greater or less part of the body. The 

 mantle usually secretes on its outer surface a calcareous 

 shell. Exceptionally, both shell and mantle may be 

 entirely absent in the adult. 



Among the Mollusca, the slug occupies the class Gastrop- 

 oda, 2 characterized by the possession of a head, which bears 

 feelers and eyes, and an unpaired foot, situated on the 

 ventral surface of the body and used to crawl upon. 3 

 Gastropods are either with or without an external shell. 

 When the shell is present, it is made of one piece, that is, 

 it is univalve^ 



1 mollis, soft. 



- -yao-Trjp, belly, stomach ; TTOUS, foot. 



3 A key to the principal families of gastropod shells of the Atlantic 

 coast of the United States is given in the Appendix, p. 174. 



4 The three orders of Gastropoda may be distinguished by aid of the 

 following key : 



!. Breathing by means of lungs ; no operculnm ; 



living on land and in fresh water . . . Puhnonnta 



a 2 . Breathing by gills ; chiefly marine. 



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