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THE POPULAR SCIENCE MONTHLY 



type corresponding with the hydroid, the macrosphseric type with the 

 medusa generation. Like these ccelenterates, the microsphaeric type 

 reproduces asexually while the macrospliEeric type reproduces sexually. 

 Like them also, the asexual generation gives rise to the sexual and the 

 latter, again, to the asexual, hence there is a typical alternation of gen- 

 erations. Like the ccelenterates, again, the sexual generation acts as a 

 nurse for the important germ plasm. Let us see how this works out in 

 the case of Polystomella crispa. 



Fig. 4. 



A fter Calkins. 



The young individual of Polystomella secretes a shell of calcium 

 carbonate and grows by feeding on various minute animals and plants. 

 Its nucleus divides by mitosis and the protoplasmic mass increases in 

 size but does not divide with the nucleus. A new shell chamber is 

 formed partly enclosing the first one. Further division of the nuclei, 

 increase of the plasmic mass and new chamber formation continues with 

 constant feeding until a typical Polystomella shell is formed, containing 

 a relatively great protoplasmic mass and hundreds of nuclei. When 

 mature, all of the nuclei save one or two break down into thousands of 



