202 



THE MET AZOA DIVISION OF LABOR 



The Hydra. Another very simple animal, which unlike the 

 sponge lives in fresh water, 1 is called the hydra. This little crea- 

 ture is shaped like a hollow cylinder with a circle of arms or ten- 

 tacles at the free end. It is found attached to dead leaves, sticks, 

 stones, or water weed in most fresh-water ponds. When disturbed 

 they contract it into a tiny whitish ball little larger than the head 

 of a pin. Expanded, it may stretch its tentacles in search of food 



almost an inch from their 

 point of attachment. The 

 tentacles are provided 

 with batteries of minute 

 darts or stinging cells, by 

 means of which prey is 

 caught and killed. The 

 outer layer of the animal 

 serves for protection as 

 well as movement and 

 sensation, certain cells 

 being fitted for each of 

 those different purposes. 

 Food Taking. The 

 tentacles then reach out 

 like arms, grasp the food, 

 and bend over with it to- 

 ward the mouth. Certain 

 cells lining the hollow 

 body cavity pour out a 

 fluid which aids in digest- 

 ing the food. Other cells with long cilia circulate the food, while 

 still other cells lining the cavity put out pseudopodia, which 

 grasp and ingest the food particles. The tentacles are hollow, 

 and the body cavity extends into them. The outer layer of the 

 animal does not digest the food, but receives some of it already' 

 digested from the inner layer. This food passes from cell to cell, 

 as in plants, by osmosis. The oxygen necessary to oxidize the 

 food is passed through the body wall, seemingly at any point, for 

 there are no organs for respiration (breathing). 



1 A few sponges, for example, spongilla, live in fresh water. 



Longitudinal section of a hydra : b, bud ; ba, 

 attached end ; m, mouth ; ov, ovary ; sp, sper- 

 mary holding sperm cells. 



