330 APPLIED BIOLOGY 



known as symbiosis, of a plant and animal is a mutually 

 helpful condition. The green plants use carbon dioxide, an 

 excretion of the animal, and make carbohydrate food, some 

 of which may be absorbed as food by the surrounding proto- 

 plasm of the animal. The oxygen freed from the carbon 

 dioxide ( 105), when that is used by the plant, may be of 

 use to the animal's cells. The green plant needs some nitrog- 

 enous food, and may get this from the animal's excretions. 

 Thus the plant gets all the materials for its food from the 

 animal, and the animal gets carbohydrate food made by 

 the plant from the animal's carbon dioxide excretion. Here 

 within a single animal we find an illustration of the relation 

 which exists between all animals and plants ( 115). 



286. Allies of Hydra : Ccelenterates. The Hydra is a 

 member of a great group, a primary division of the animal 

 kingdom, known as Coelenterata, or popularly called ccelen- 

 terates. The name is a combination of ccelome (the technical 

 name for body-cavity in all animals), and enteron (technical 

 name for a digestive cavity), and was originally given be- 

 cause it was supposed that in Hydra and its allies the one 

 cavity (the digestive cavity) appeared to combine both the 

 digestive cavity and the body-cavity of higher animals. 

 This was wrong, for in the frog the body-cavity is between 

 the digestive cavity (stomach and intestine) and the body- 

 wall. The cavity in Hydra simply corresponds to the interior 

 of the frog's stomach and intestine, and if it had a space 

 between its endoderm and ectoderm this space would corre- 

 spond to the body-cavity in a higher animal. 



The most characteristic features of the ccelenterates are 

 shown in Hydra; namely, two layers of cells, a digestive 

 cavity, and nettle-cells. Animals of other groups have 

 tentacles, but only ccelenterates have nettle-cells on their 

 tentacles. . 



287. Hydroids. Imagine the buds on a hydra remaining 

 attached, as do buds on trees, and budding repeatedly, and 



