122 COLLEGE ZOOLOGY 



formed. The gastrula-like structure soon becomes ciliated and 

 elongates into a free-swimming larva called a planula (Fig. 73, C). 

 This soon acquires a central cavity, becomes fixed to some object, 

 and proceeds to found a new colony. 



b. Metagenesis 



Metagenesis is the alternation of a generation which repro- 

 duces only asexually by division or budding with a generation 

 which reproduces only sexually by means of eggs and spermato- 

 zoa. This phenomenon occurs in other groups of the animal 

 kingdom, but finds its best examples among the ccelenterates. 

 Obelia is an excellent illustration of a metagenetic animal. The 

 asexual generation, the colony of polyps (Fig. 73, A), forms buds 

 of two kinds, the hydranths and the gonangia. The medusae 

 (Fig. 73, B), or sexual generation, reproduce the colony by 

 means of eggs and spermatozoa. 



The polyp and medusa stages are not equally important in all 

 Hydrozoa; for example, Hydra has no medusa stage and Geryonia 

 no polyp or hydroid stage. Various conditions may be illustrated 

 by different Hydrozoa. In the following list, O represents the 

 fertilized ovum, H, a polyp, M a medusa, m an inconspicuous or 

 degenerate medusa, and h an inconspicuous or degenerate polyp. 



1. — H — O — H — {Hydra). 



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



3. — H — M — O — H — M — O (Obelia). 



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



5. — M — O — M — O (Geryonia). 



c. A Jellyfish or Medusa — Gonionemus 



The structure of a hydrozoan jellyfish or medusa may be illus- 

 trated by Gonionemus (Fig. 74). This jellyfish is common along 

 the eastern coast of the United States. It measures about half 

 an inch in diameter, without including the fringe of tentacles 

 around the margin. In general form it is similar to the medusa 

 of Obelia (Fig. 73, B). The convex or aboral surface is called 



