Fig. 42. Vorticella 



This one-celled ani- 

 mal lives in water, 

 attached by its stalk 

 to a rock or twig. 

 When disturbed it 

 contracts the stalk 

 and "bell "suddenly 



Fig. 43. Diagram of sponge structure 



A sponge is a colony of cells arranged about hollow spaces, 

 a, which are connected with the surrounding water by 

 means of hollow channels, b, carrying currents inward, and 

 by means of other channels, c, carrying currents outward 

 through larger tubes, or "sewers," d. The currents are 

 produced by the constant vibration of cilia projecting into 

 the spaces, and they bring to the cells fresh supplies of 

 food and oxygen, and carry away waste 



Fig. 44. The jellyfish Aiirelia 



The mature medusa, a, reproduces sexually, the gametes being thrown into the water, 

 where fertilization takes place. The egg develops into an individual having the gen- 

 eral form of a hydra, b, and attaches itself to a rock. The animal elongates and 

 breaks up into a number of individuals by means of constrictions, so that it comes 

 to resemble a pile of bowls. Each individual, when separated, turns over and swims 



away, changing into a medusa, a 



