176 INTRODUCTION TO ZOOLOGY. 



the words of Professor Jones, " There will be (3,000 X 20 X 6 ) 

 360,000 of these microscopic suckers upon the head of one 

 Clio; an apparatus for prehension perhaps unparalleled in 

 the creation." 



II. GASTEROPODA. 



Fig. 163. VOLUTE (THE ANIMAL REPRESENTED IN MOTION). 



IF we look at the common Snail, as it crawls along, we notice 

 that the only organ it possesses as a substitute for legs is a 

 broad muscular disc, forming the lower surface of the body. 

 Hence the compound terra Gasteropoda (belly-footed) indicates 

 the peculiarity of its locomotive structure, and is used as 

 the name of the class in which a similar structure prevails 

 (Figs. 147, 153, 163). 



The class is extremely numerous, and is conveniently dis- 

 tributed into orders distinguished by modifications of their 

 respiratory organs.* Into any minute details of these structural 



* It may be convenient to enumerate, in one place, the orders into 

 which the class is divided, accompanied by an explanation of the 

 scientific names. 



Nudibranchiata gills naked. 



Inferobranchiata gills inferior or lower. 



Cyclobranchiata gills round the body. 



Tectibranchiata gills covered by mantle. 



Pulmonata breathing by lungs. 



Scutibranchiata gills with a shield. 



Tubulibranchiata gills with a tube. 



Pectinibranchiata gills like a comb. 



The order last mentioned is the highest in point of organization; in 

 it the sexes are distinct. 



