MICROSCOPE AT THE SEA-SIDE. 



79 



This structure resembles the tubular structure ID 

 the teeth of the higher animals. 



Fish-scales are frequently permeated with minute 

 tubes, drawn in figure 177, plate 6. These appear 

 to be the work of some minute parasite, such as 

 that producing the tubules in shells, and which has 

 hitherto evaded the scrutinizing investigation of 

 the microscopic observer. 



The fishes of the earlier rocks belong to the 

 ganoid and placoid groups. The great majority of 

 recent fishes belong to the remaining groups. The 

 common sole affords an instance of the ctenoid, or 

 comb-like scale, seen at figure 178. 



The cycloid, or circular scales, are found in such 

 fish as the whiting, and represented at figure 179. 

 It is not uncommon to find in these scales cal- 

 careous particles, shown at a. In the sprat the 

 cycloid scale assumes a form almost as broad as it 

 is long, and is seen in figure 180. 



The examination of these hard structures in the 

 marine creatures is a good preparation for the 

 further study of those hard parts in the higher 

 animals to which the name of bone and ivory is 

 given. Such things may, however, be procured in 

 the house ; and when the rain is falling, the sea- 

 side forsaken, or the country miserable-looking, we 

 can still enjoy the long winter evenings with our 

 Microscope in the house. 



