POLYPS {CUPLIKE ANIMALS) 27 



In what ways does the hydra show ''division of labour"? Answer 

 this by explaining the classes of cells specialized to serve a different 

 purpose. Which cells of the hydra are least specialized? In what par- 

 ticulars is the plan of the hydra different from that of a simple sponge ? 

 An ingenious naturalist living more than a century ago, asserted that it 

 made no difference to the hydra whether the ectoderm or the endoderm 

 layer were outside or inside, — that it could digest equally well with 

 either layer. He allowed a hydra to swallow a worm attached to a 

 thread, and then by gently pulling in the thread, turned the hydra inside 

 out. More recently a Japanese naturalist showed that the hydra could 

 easily be turned inside out, but he also found that when left to itself 

 it soon reversed matters and returned to its natural condition, that 

 the cells are really specialized and each layer can do its own work and 

 no other. 



Habits. — The hydra's whole body is a hollow bag, the 

 cavity extending even into the tentacles. The tentacles 

 may increase in number as the hydra grows but seldom 

 exceed eight. The hydra has more active motion than 

 locomotion. It seldom moves from its place, but its ten- 

 tacles are constantly bending, straightening, contracting, 

 and expanding. The body is also usually in motion, bend- 

 ing from one side to another. When the tentacles ap- 

 proach the mouth with captured prey, the mouth (invisible 

 without a hand lens) opens widely, showing five lobes or 

 lips, and the booty is soon tucked within. A hydra can 

 swallow an animal larger in diameter than itself. 



The endoderm cells have amoeboid motion, that is, they 

 extend pseudopods. They also resemble amoebas in the 

 power of intra-cellular digestion ; that is, they absorb the 

 harder particles of food and digest them afterwards, re- 

 jecting the indigestible portions. Some of these cells have 

 flagella (see Fig. 39) which keep the fluid of the cavity 

 in constant motion. 



Sometimes the hydra moves after the manner of a small 

 caterpillar called a " measuring worm," that is, it takes 

 hold first by the foot, then by the tentacles, looping its 



