168 ANARTHROPODA 



Ccelenterata : Hydroids, Jellyfish, and Sea Anemones 



With the exception of a few species all of the Ccelenterata 

 live in salt water. They differ from most of the animals 

 thus far described in having no body cavity (ccelome) sur- 

 rounding the digestive canal and in the presence of only 

 two layers of cells in the body wall. 



Hydra Viridis. This common freshwater polyp may be 

 found in the summer and autumn by placing in several 

 glass jars or aquaria a considerable quantity of duck-meat 

 or duck-weed, together with the water in which it is found 

 growing. This plant is a favorite support for the animals. 

 Place the jars in a north or east window and examine them 

 daily for a week, looking through the water toward the 

 light for a minute creature a fourth of an inch long or less 

 with tentacles. It is usually attached to the side of the 

 jar toward the light. When found it may be transferred 

 with a pipette (medicine dropper) to a watch crystal for 

 study and covered with a few drops of water. 



The structure of the Hydra is that of a simple tube with 

 walls made with two layers of cells, the outer of which is 

 the ectoderm and the inner the endoderm. For securing 

 food there are around the mouth several tentacles bearing 

 stinging cells (nematocysts) containing coiled, whiplike 

 processes which dart forth at the touch of an animal and 

 numb it. The tentacles then slowly work the prey through 

 the mouth into the digestive cavity where digestive juices 

 secreted by some of the cells render parts of it absorbable 

 by certain cells of the endoderm. The undigested portions 

 are cast out through the mouth. Water fleas and algae 



