NUTRITIVE ZOOIDS 

 MOUTH 



The Animal Kingdom - 633 



MEDUSAE 



GONADS - 



PART OF 

 COLONY 



DEVELOPING COLONY 



Fig. 32-9. Obe//o. This hydrozoan shows a regular alternation of sexual and asexual generations— a rather 

 rare phenomenon in the Animal Kingdom. (By permission, from Genera/ Zoology, by Tracy I. Storer. McGraw- 

 Hill Book Co., Inc.) 



form of a small jellyfish, which is called a 

 medusa. 



Class 2. The Scyphozoa (True Jellyfishes). 

 These coelenterates resemble the medusa of 

 Obelia, except that they usually are larger 

 and have a greater abundance of gelatinous 

 mesoglea. Also the Scyphozoa do not have 

 any velum, the sheet of tissue that partially 

 closes off the subumbrellar space in hydro- 

 zoan medusae (see Fig. 32-9). A hydroid form 

 appears very transiently during embryonic 

 development. 



The multitude of stinging cells on the long 

 tentacles of some of the larger Scyphozoa may 

 represent a painful hazard for unwary swim- 

 mers. The largest kind of jellyfish, Cyanea, 

 is a blue and orange giant with a diameter 

 up to 12 feet and tentacles up to 90 feet in 

 length. However, this species is rarely seen 

 along the Atlantic or Pacific coasts of North 

 America, except perhaps in the most north- 

 erly parts. 



Class 3. The Anthozoa (Sea Anemones and 



Corals). No medusoid stage is present. The 

 general form is somewhat suggestive of 

 Hydra, except much more complex. A gul- 

 let, which resembles an inverted hypostome 

 (Fig. 16-4), is always present. Also there are 

 sheets of tissue called septa, which pass radi- 

 ally inward from the body wall, subdividing 

 the enteric cavity and increasing its digestive 

 capacity. Colonial forms are frequent. 



The anemones produce no skeletons but 

 the closely related corals often produce elab- 

 orate supporting structures at the basal (at- 

 tached) end of the body (Fig. 32-10). These 

 may be predominantly calcareous, as in the 

 case of the red jewel coral, Corallium; or 

 they may be composed of a horny protein, 

 gorgonin, as in the case of the beautiful sea 

 fan, Gorgonia. 



Some of the colonial corals, which are ex- 

 tremely abundant in warmer (20° C and 

 over) parts of the oceans, have played a domi- 



