GEOGRA PHICA L D IS TRIE U TION. 



131 



well-developed tongue {radulci), which is generally armed 

 with minute denticles set in rows (Fig. 34). The other types 



-si 



Fig. 33. — A Gastropod, the common whelk, Buccinuiit undafuin, showing the spiral shell on the 

 back of the animal, its large flattened foot, distinct head with two tentacles, at the base of 

 which are the eyes. The siphon si and the opt rculum c/ are special parts not found in all 

 Gastropods. 



Fig. 34.— Examples of the dentition of Gastropoda, single transverse rows of the denticles of the 

 lingual ribbon {raiiu/a), greatly magnified, of (A) Natica, (B) Nassa, (C) Pleurotoma, (Z?) 

 Scalar ia. 



of Glossophora are adjusted to various conditions of en\-iron- 

 ment, but for our purpose it will be better to confine our 

 attention at present to the single type of the class Gas- 

 tropoda. 



Distinguishing Characters of the Class Gastropoda. — The com- 

 mon external characters of all Gastropods are these, viz. : 

 Head and sense organs well developed, the former often 

 bearing tentacles; a ventral muscular foot and undivided 

 mantle, which frequently secretes a plate-shaped, or spirally 

 twisted shell. The paleontologist knows Gastropods by their 

 calcareous, more or less spirally twisted, univalve shells. 

 These Gastropods, of which several tens of thousands of 

 species are described, are specifically adjusted to all kinds of 

 conditions of environment, and are distributed from the bot- 

 tom of the ocean to the tops of the mountains. 



