316 Descriptive Zoology. 



The Endoderm. The cells of the endoderm are larger 

 than those of the ectoderm. They are also less uniform 

 in size. They are darker colored, sometimes containing 

 brown coloring matter. The green hydra contains chloro- 

 phyll. The endoderm cells often exhibit amoeboid move- 

 ments, and frequently show food particles and large 

 contractile vacuoles, such as noticed in amoeba. Project- 

 ing from these cells into the cavity are sometimes found 

 large flagella. 



Digestion in Hydra. When food is taken into the central 

 cavity it is supposed to be partly digested here, this often 

 being called the digestive cavity. But, at any rate, parti- 

 cles not completely digested are taken into the endoderm 

 cells, where no doubt digestion takes place ; and from the 

 material digested by the endoderm cells the ectoderm gets 

 its nourishment. We here see a division of labor, the outer 

 layer producing the motion, obtaining the food, and pro- 

 tecting the whole, while the inner cells do the work of 

 digestion. 



Locomotion. While the hydra is pretty firmly attached 

 by means of a sticky substance secreted by the cells of the 

 base, it can let go and move away. It sometimes bends 

 over, attaches itself by the tentacles, and then lets go at 

 the base and pulls the base up close to the place where th( 

 tentacles are fastened, and by repeating this action crawl 

 along like a "measuring worm." Or it may bend ovei 

 attach itself by the tentacles, let go at the base, and turn 

 the base clear over, thus turning a complete somersault, 

 though slowly, instead of with a spring. Sometimes, also, it 

 appears to crawl slowly by means of the tentacles alone. 

 But hydra is not a great traveler, preferring to wait for 

 something to turn up, rather than hunt for food. We see 

 how well fitted it is for a sedentary life, for with the long, 



