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 meduse. 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 medusew. They 
