218 THE OPEN BOOK OF NATURE 
young, and so on. At this rate of reproduction, 
which is a very moderate estimate, one would expect 
Cyclops, small as they are, to fill a pond in a very 
short time ; but these animals have plenty of enemies 
in their native haunts—enemies who consume them 
wholesale, and thus maintain the balance of Nature. 
I have been looking in our tubes for a creature that 
can demolish both Water Fleas, Cyclops, and even 
small Worms, but I’m afraid we have not secured a 
specimen ; so you will have to be content with a 
description, and live in expectation of capturing 
one another day. I refer to the Hydra. 
The Hydra is an animal of a most astonishing 
kind, possessing an almost superhuman vitality. A 
good specimen may be nearly an inch long, so it 
can be observed with the naked eye ; but its details 
are observed more clearly when examined with a 
hand-magnifier, or under a very low power of the 
microscope. Three species of Hydra may be found 
—the Green (Hydra viridis), the Orange-Brown 
(H. vulgaris), and the Brown (H. fusca). What I 
have to say applies particularly to the green species. 
The creature seems to have a body of a green jelly- 
like substance. It attaches itself by a foot to water- 
weeds. Its body is tubelike, or cylindrical, sur- 
mounted at the head by some six or more arms or 
tentacles arranged in a starlike manner. At the 
end of the tubelike body, and surrounded by the 
tentacles, is the mouth. When the animal is active 
it stretches its body to full length, and waves its 
tentacles in all directions in the effort to secure food. 
If anything good to eat—say a Water Flea or Cyclops 
—comes within reach of the tentacles, it is seized 
