THE FISHES, MOLLTJSCA, AND OTHEB OBJECTS. 



47 



and when the existing German ocean was the site of 

 lakes, &c. The prepared legs of this beetle are good 

 objects for the microscope, either for transmitted or 

 polarised light. Whilst searching for things that 

 can thus be turned to use, do not let us forget the 

 common Water Fleas, whose antiquity is greater than 

 that of any object we have Fig. 41. 



yet mentioned. The shales 

 overlying many of our coal 

 seams are quite fissile, owing 

 to the myriads of fossil cases 

 of these creatures which 

 strew their surfaces, and 

 thus cause them to split up 

 readily. In every fresh- 

 water deposit, of every geo- 

 logical age, you find remains 

 of fossil water-fleas ; and it 

 is surprising how little the 

 general type has altered 

 through the thousands of 

 centuries that must have 

 elapsed since they were 

 called into existence ! The Daphnia pulex (male) ' 

 living forms are quite visible to the naked eye, but 

 it requires optical aid to bring out their individual 

 beauties. You may be certain of finding several 

 species in any decent pond or tarn. The commonest, 

 perhaps, is Daphnia pulex the typical water-flea 

 (Figs. 41, 42). The following illustrations show them 



