THE FISHES, MOLLUSCA, JLND OTHER OBJECTS. 59 



tubercles with which the tentacles are thickly beset. 

 These are called " urticating threads," and the animal 

 seems to have the power of thrusting them forth at 

 pleasure. "Ry pressing the tentacles of a hydra 

 between two pieces of glass, Mr. Fullagar forced 

 these nettling or "urticating" threads out. The 

 tentacles are rendered very powerful weapons of 

 retention by means of the three recurved booklets 

 attached ko them. Myriads of these arrow-headed 

 stings are crowded on the tentacles, and yet the 

 full grown size of the little hydras rarely exceeds 

 a quarter of an inch ! 



One can hardly wonder at the revolution which 

 has taken place in natural science within the last 

 thirty years, when we consider how that wonderful 

 instrument, the microscope, has opened our eyes upon 

 the fulness of the animal kingdom. Even to glance 

 at the multitudinous objects which this tarn could 

 afford at the Algae which mantle its shallower 

 surface, or the Desmids which might be skimmed off 

 its mud would occupy a little volume. The latter 

 are among the most elegant of natural forms, and 

 their ornamentations might be studied for the arts 

 with considerable profit. Perhaps on the same 

 leaves or stalks of water plants as those where you 

 secured the hydras, your sharp and trained eyes may 

 detect those even more wonderful, because more 

 highly organized creatures, the Kotifers, or "wheel 

 animalcules." Many of the wheel animalcules, 

 however, are free swimming, active little animals, 



