222 AN INTRODUCTION TO ZOOLOGY. 



fied respectively as Sqjhonostomata (siphon-mouthed) 

 and Holostomata (plain- mouthed). The notch is a 

 feature which is only indirectly associated with the 

 nature of the animal's food, since the siphon for which 

 it provides room is a fold of the mantle which is 

 respiratory in function. The association of the notch 

 with the distinction in food habits is not quite invari- 

 able, but for the shell-collector the classification is a 

 very convenient one. The student should note that 

 the small end of the spiral is of course the posterior 

 end of the shell : in placing shells in a cabinet, there- 

 fore, the elongated shells, such as olives and harps, if 

 placed, as they often are, with the small end away 

 from the spectator, are turned the reverse way from 

 the snail shells and other flattened kinds, which are 

 seen to the best advantage by turning the small end 

 towards the spectator ; while in all cases it must be 

 remembered that the axis of the spiral of the shell is 

 necessarily at an angle with the direction of the living 

 animal in crawling. Many of the Gasteropoda have a 

 stopper to close the shell with, when the animal retires 

 inside. This stopper is called the operculum (lid). 

 Some of these stoppers are calcareous, others horny : 

 they exhibit in many cases a spiral mark correspond- 

 ing with the gradual increase in size necessary as the 

 mouth of the shell widens, or in other cases concentric 

 marks, or complications of both. 



In some kinds, the shell undergoes considerable 

 modification in its adult form. When it ceases to in- 

 crease in the length of its whorl, the lip may become 



