I20 COELENTERATES 



(5) In the reproduction of the hydra, the egg leaves the 

 body surrounded with a shell-like secretion of its covering. 

 In this condition the egg sinks to the bottom of the 

 water and may remain for sometime before developing 

 further. After a lapse of considerable time, if conditions are 

 favorable, the animal develops into an adult hydra. By 

 this power of remaining inert, the hydra can pass success- 

 fully through vicissitudes of coldness and dryness that 

 would otherwise destroy it. 



SUMMARY. 



The structure of the hydra is very simple. Its body 

 consists of an outer layer of tough cells which is called 

 ectoderm, and an inner layer of softer cells, endoderm. 

 Between these two layers is a row of scattered, poorly de- 

 veloped cells, called mesogloea. In the process of growth 

 from the egg, the outer primitive layer called blastoderm 

 is folded at one point producing a saclike interior like the 

 folding in of one side of a hollow rubber ball. From this 

 the animal develops into the adult form with its tough 

 ectoderm on the outside and the hollow digestive tract, sur- 

 rounded by the cells of the endoderm, on the inside. 



The sea anemone is a higher form with a more compli- 

 cated though similar structure. Its development from the 

 egg is similar to that of the hydra, but the mesog'loea is so 

 developed that it forms muscles on the sides of the mes- 

 enteries. Indeed the mesogloea is so well developed here 

 that it perhaps deserves the name that it receives in the high- 

 er animals, viz. : mesoderm. Find the meaning of these 

 words. 



The animals that secrete coral are called coral polyps, 

 polyp being the name applied to each individual of the col- 



