142 AN INTRODUCTION TO ZOOLOGY 



is more pronounced in some species than in others, e.g., Hydra 

 has no medusa stage at all; whereas certain species have no 

 polyp stage but pass their entire existence as medusae. Various 

 conditions may be illustrated by different Hydrozoa. In the 

 following table, O represents the fertilized ovum; H, a polyp; 

 M, a medusa; m, an inconspicuous or degenerate medusa, and 

 h, an inconspicuous or degenerate polyp (103). 



1. O H O H O (Hydra). 



2. O H m O H m O (Sertularia). 



3. O H M O H M 0(0belia). 



4. O h M O h M O (Liriope). 



5. O M O M O (Geryonia). 



The alternation of a sexual with an asexual generation, as in 

 examples 2, 3, and 4, just listed, is known as metagenesis. This 

 phenomenon occurs in other groups of the animal kingdom, but 

 finds its best examples among the Ccelenterates. 



Division of Labor among Coelenterates. Not only have the 

 somatic cells of the Coelenterates become differentiated into ecto- 

 derm and entoderm, in each of which cells may be recognized 

 having particular functions to perform; but in certain groups 

 colonial species are found in which the various members of the 

 colony are so specialized for certain kinds of work, that they are 

 incapable of carrying on other processes. Perhaps the best 

 example of such a condition is Physalia, the " Portuguese Man- 

 of-War " (Fig. 67). Physalia is a colonial Hydrozoan consist- 

 ing of a large float (pn.) with a sail-like crest (cr.) from which a 

 number of polyps hang down into the water. Some of these 

 polyps are nutritive, others are tactile; some contain batteries 

 of nematocysts, others are male reproductive zooids, and still 

 others give rise to egg-producing medusae. 



Corals. One group of Ccelenterates, the corals, is of especial 

 interest because of its economic importance. The corals are 

 found principally in the tropics. They live near the shore, which 

 not infrequently is built up of coral skeletons. The hard parts 



