58 A HALF-HOUR WITH THE 



tubes (Fig. 173, PL 6). These appear to be the 

 work of some minute parasite which has hitherto 

 evaded the scrutinizing investigation of the micro- 

 scopic 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 (Fig. 174, PL 6). 



The cycloid, or circular scales, are found in such 

 fish as the whiting (Fig. 175, PL 6). It is not 

 uncommon to find in these scales calcareous par- 

 ticles (a). In the sprat the cycloid scale assumes 

 a form almost as broad as it is long (Fig. 176, 

 PL 6). 



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, and the 

 sea-side forsaken, and the country miserable-look- 

 ing, we can still enjoy the long winter evenings 

 with our Microscope in the house. 



