MICROSCOPE AT THE SEA-SIDE. 57 



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 pro- 

 jecting points, the two outer of which are larger 

 than the inner (Fig. 167, PL 6). In the limpet, the 

 teeth present four projections, which are all of the 

 same size (Fig. 168, PL 6). In the common peri- 

 winkle another kind of arrangement is observed 

 (Fig. 169, PL 6). 



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 (Fig. 170, PL 6) will 

 give a lively idea of the nature. of the scavenging 

 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 

 which they belong. The sturgeon has ganoid scales. 

 They are shiny, and have a structure like bone 

 (Fig. 171, PL 6). 



The sharks, rays, and skates have placoid scales. 

 They are frequently terminated with a prickle, as 

 in the scales of the skate (Fig. 172, PL 6). This 

 structure resembles the tubular structure in the 

 teeth of the higher animals. 



Fish-scales are frequently permeated wifch little 



