156 INTRODUCTION TO ZOOLOGY. 



The hydra has a special provision for capturing 

 small animals which constitute its food. Certain cells 

 in the tentacles are provided with a coiled, spring- 

 like thread terminating in a harpoon point. When a 

 small animal like Cypris or Daphnia comes near the 

 tentacles, the little harpoons dart out from the nemat- 

 ocysts, or lasso cells, and stick into its body. This 

 paralyzes the animal, which is then drawn by the 

 tentacle to the mouth. 



The hydra is a notable example of the ability to 

 reproduce lost parts. It may be cut into two pieces, 

 and each piece will become a hydra. It may be 

 turned wrong side out, and what was before the 

 external surface will take on the function of digestion. 

 If all the tentacles are cut off, new ones will grow out. 



Hydras reproduce by budding, and by eggs. In 

 the case of budding, a small bud grows out on one 

 side of the body, which gradually develops tentacles, a 

 mouth, and a body cavity. Finally it separates from 

 the parent and attaches itself to another support. 



Hydrpids are very abundant in shallow Water along 

 the sea shore. They look like plants, branching 

 freely, and each branch being tipped with a flower- 

 like body, somewhat similar to the hydra. All the 

 hydranths, or hydra-like bodies, that grow from a 

 common stem constitute a colony of individuals 

 which are dependent upon each other. What one 

 eats, nourishes all. In a case like this, what shall we 

 call the individual? Is it the single hydranth, or is 

 it all the hydranths that grow from a common stem? 

 The budding of the hydra manifests the same thing. 

 If the young hydra did not separate from the parent, 

 should we have one individual or two? It is not 

 always easy to tell what constitutes an individual. 



Hydroids manifest the phenomenon of alternation 

 of generations. Some hydroids give rise to jelly fishes, 

 and the eggs laid by some jelly fishes produce hydroids. 



