Page:Popular Science Monthly Volume 79.djvu/576

572 type corresponding with the hydroid, the macrosphæric type with the medusa generation. Like these cœlenterates, the microsphæric type reproduces asexually while the macrosphæric 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 generations. Like the cœlenterates, 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.

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