78 A HALF-HOUR WITH THE 



croscopic examination. All that is necessary to 

 examine these organs is, to take the palate, or 

 tongue, as it is called, of any of our common mol- 

 luscs, and to stretch it on a glass slide, when it may 

 be seen by transmitted or reflected light. In the 

 common whelk, the teeth are placed in rows, and 

 are composed of a broad base with four projecting 

 points, the two outer of which are larger than 

 the inner, as seen in figure 171, plate 6. In the 

 limpet, the teeth present four projections, which 

 are all of the same size ; seen in figure 172. In 

 the common periwinkle another kind of arrange- 

 ment is observed, and is figured at 173. 



When sea-side specimens have been observed and 

 put up, the fresh-water mollusca may be next 

 investigated. Here other forms will be observed. 

 The species of the genus Limneus are found 

 in every pond, and kept in every aquarium. 

 The tongues of these creatures, represented at 

 figure 174, will give a lively idea of the nature 

 of the scavengering processes they carry on . 



The scales of fishes are interesting microscopic 

 objects. The structure of these organs indicates 

 the family of fishes to. which they belong. It is in 

 this way that a single scale found in a rock will 

 throw a light on the nature of the fishes which 

 inhabited the seas or rivers from which the rock 

 was deposited. 



Fishes' scales have been called ganoid, placoid, 

 cycloid, and ctenoid, according to the families to 

 \vhich they belong. The sturgeon has ganoid 

 scales. They are shiny, and have a structure 

 like bone, and are represented at figure 175, 

 plate 6. 



The sharks, rays, and skates have placoid scales. 

 They are frequently terminated with a prickle, as 

 in the scales of the skate ; seen at; figure 1 76. 



