REPRODUCTION 



65 



which are automatically discharged when the tentacles 

 come in contact with little animals. The darts stun the 

 prey and render escape impossible. The tentacles sur- 

 round the food and carry it to the mouth, which opens 

 directly into the food cavity. The action of the tentacles 

 in doing this work sug- 

 gests the idea that each 

 tentacle has some way 

 of realizing the efforts 



Endoderm 



Mesoglea 



Ectoderm 



Figure 63. — Diagram. 

 To explain cell layers in Figure 62. 



of the others. 



We should keep in 

 mind that in the meta- 

 zoan the united cells are 

 in connection with each 

 other through the cell 



walls. This is true even if we are not able to trace the 

 connections with the microscope. In the higher animals 

 we shall find that connections between cells are made by 

 means of nerve cells. The development of a nervous 

 system only carries out division of labor to a greater 

 degree. 



50. Respiration and Excretion. — By osmosis, oxygen is 

 absorbed from the water by the cells of the ectoderm. 

 The water that enters the mouth carries oxygen, and by 

 osmosis it is absorbed by the cells of the endoderm. At 

 the same time the carbon dioxide from the cells is thrown 

 off into the water. 



51. Reproduction. — The hydra reproduces both sexually 

 and asexually. In sexual reproduction eggs and sperms 

 are produced by the ectoderm cells. The sperm cells 

 escape into the water and, like sperm cells of all other 

 animals, have the power of locomotion. The fusion of 

 the egg cell and a sperm cell starts growth which results 

 in the division of the egg cell into many other cells. 



