THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 43 



to form distinct sense organs. The muscle processes are 

 better developed, and the digestive cavity is differentiated 

 into central and peripheral portions. In these dimorphic 

 polyps the fixed polyp individuals reproduce by the simple 

 way of budding, while the medusa individuals reproduce 

 by producing special reproductive cells of two kinds, which 

 must fuse to form a cell capable of developing into a new 

 polyp. 



25. Corals. There are many kinds of polyps and jelly- 

 fishes, and they present a great variety of shape and size 

 and general appearance. Many polyps exist only in the 

 true polyp form, never producing medusae. Others have 



FIG. 21. A polyp, or sea-anemone (Metridium dianthus). 



only the medusa form. Some live in colonies, and others 

 are always solitary. The animals we know as corals are 

 polyps which live in enormous colonies, and which exist 

 only in the true polyp form, not producing medusae. They 



