64 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



the little tree-like hydroids that are found on the submerged 

 rocks or shells along the shore at all depths. These little 

 animals look so plant-like that they are often called zoo- 

 phytes, or plant-animals, a name that is also applied to other 

 polyps. Many of them however live a part of their lives as 

 active, free-swimming forms before they settle down to be- 

 come attached to the places where they are to remain. In 

 the shallow waters of all tropical seas the coral polyps grow in 

 such numbers that their myriad skeletons form whole groups 

 of little islands and great projecting barrier reefs that may 

 reach for miles along the shores. The great barrier reef off 

 the north shore of Australia is more than a thousand miles 

 long. Only a few of the polyps live in fresh water. Hydra, 

 which is found in nearly all fresh water ponds or streams, is 

 the most familiar example, and has already been studied. 



Hydroids and Jelly-fishes. In each of the four groups, or 

 classes, into which the polyps and jelly-fishes are divided there 

 are many modifications of the simple type plan presented by 

 Hydra. If the buds that develop on Hydra should remain 

 attached and continue to grow and in turn produce other buds, 

 there would soon be developed a tree-like colony with a Hydra- 

 like animal at the end of each branch. Some marine animals 

 are developed in just this way, and are known as hydroids. The 

 class Hydrozoa (Gr. hydor, water; zoon, animal) to which these 

 belong includes the Hydra and other Hydra-like animals. The 

 Hydrozoan colonies may be made up of two kinds of in- 

 dividuals, or zooids. One kind, known as the nutritive zooids, 

 or polyps, retains much of the general appearance of the 

 Hydra; the other kind, known as the reproductive zooids, or 

 medusae, develop into quite different looking animals. The 

 medusas are produced by budding off from the bases of the 

 polyps. They are umbrella-shaped, jelly-like masses and are 

 commonly known as jelly-fishes. After budding off from the 

 polyps they float and swim in the water for some time and 

 finally produce egg cells and sperm cells. From each fer- 

 tilized egg a new individual develops. This new individual 

 is at first an active free-swimming larva, called planula, which 

 does not resemble either a medusa or a polyp. After a while 



