THE INVERTEBRATES 



135 



very small compared with most animals we know, it is much 

 larger than any of the Protozoa, being when expanded 

 nearly one-fourth of an inch long. It is also not composed 

 of a single cell but of hundreds of cells. It is one of the 

 simplest of the many-cell- 

 ed animals, i.e., Metazoa. 

 Hydra may be found at- 

 tached to bits of sticks, 

 stones, and leaves in pools 

 not too stagnant. There 

 are two common kinds, 

 one brown and one green. 

 Specimens should be 

 brought into the school- 

 room alive, and kept in 

 a dish of water in the 

 light. To observe the 

 habits of Hydra, examine 

 a live specimen, attached 

 to a bit of leaf or stick, in 

 a watchglass, under the 

 low power of a compound 

 microscope, or with a good 

 magnifier. 



Note the cylindrical 

 body, attached at its base, 

 and with a series of ten- 

 tacles projecting from its 

 free end. How many ten- 

 tacles are there? They arise 



FIG. 55. Hvdra; note two tentacles 



j 



catching an insect larva; note the 

 budding young Hydra. (Natural 

 size, one sixth of an inch high; 

 from life.) 



in a circle 



aoout the 



mouth. Have some small water-fleas in the water 

 and observe Hydra's method of catching and eating food. 

 Note that when it captures one of the water-fleas with its 

 tentacles the flea soon ceases to struggle. It is paralyzed. 

 On the tentacles are many extremely fine, little, stinging 



