CCELENTERA TES. 



17 



CCELEXTERATES. 



The term Coelenterates is applied to the group of animals which in- 

 cludes corals, jelly-fish, sea-anemones, and the like, from the fact that the 

 digestive and the body cavities are not distinct. Formerly these animals 

 were united with the next group (the Echinoderms), under the name Radi- 

 ates, but it has long since been shown that they have almost nothing i n 

 com lion. A still older name is zoophytes, or '•animal-plants:* which, when 

 applied to many of the species, is very apt. for they are superficially much 

 like some of the sea-weeds, and are often collected with them. 



W ith about half-a-dozen exceptions all of the Ccelenterates are marine, 

 but one of the fresh-water forms is very interesting. It 

 is the fresh- water hydra. It is fond of stagnant or semi- 

 stagnant water, and at times is very abundant, attached 

 to plants or to the glass of aquaria. In color it may 

 be green or a dirty reddish brown, while a specimen half 

 an inch in length is a large one. At one end of the body 

 is the mouth, and surrounding it are from four to seven 

 long, slender, flexible, tapering arms, or tentacles. It is 

 with these arms that the hydra obtains its food. As the 

 animal hangs in the water these tentacles are extended 

 in every direction, keeping up a slow but constant waving 

 motion. When some small animal, for instance a cyclops, 

 swimming through the water, comes in contact with one 

 of these arms, it stops instantly, as if paralyzed. The 

 arm now slowly curls around the victim, and lazily con- 

 veys it to the mouth. 



Why does the prey stop so short ? It is really para- 

 lyzed, and the method by which this is accomplished is ex- 

 tremely interesting. Scattered over the arms are bunches 

 of peculiar cells, much larger than their neighbors. In- 

 side of each is a long, hollow filament, and a quantity of 

 fluid, closely similar to, if not identical with, formic acid, 

 and at the slightest touch the filament shoots out and 

 penetrates the body of the victim. Then the contents 



Fio. l»i. — Fresh-water 

 hydra, with (a and b) 

 young budding from 



it. 



