COELENTERATA AND CTENOPHORA 



The genera Cyanea and Aurellia are representative Scyphomedusae found in 

 North Atlantic waters. In typical cases the life cycle of a scyphomedusa 

 consists of the following sequence (Fig. 10.12): a planula larva develops from 

 a zygote; this larva produces an attached polyp generation, the scyphistoma, 

 from which free-swimming medusae arise in succession by transverse budding 

 or strobilization. The young medusae released from the strobila are saucer- 

 like individuals called ephyrae; they grow and transform into adult, sexually 

 reproducing jellyfishes. Asexual reproduction of the polypoid generation, by 

 budding to produce additional polyps, has also been reported. 



Fig. 10.12. Structure and life cycle of Scypho- 

 medusae. A, polyp stage, or scyphistoma, of 

 Cyanea, developed from a planula larva, and 

 two strobilas undergoing asexual reproduction, 

 giving rise to many ephyrae. B, ephyra, or 

 immature medusa, probably of Chrysaora. C\ 

 mature jellyfish or medusa stage of Aurellia. 

 The bell of this medusa is traversed by many 

 gastrodermal canals, shown in only one quad- 

 rant; the margin bears many short tentacles as 

 well as marginal sense organs. The gonads are 

 prominent semicircular structures borne be- 

 neath the radial canals; the oral tentacles are 

 long and flexible and are the structures chiefly 

 used in feeding. {A, redrawn from J. J. 

 Steenstrup, Ueber den Generationswechsel , Copen- 

 hagen, 1842. Steenstrup was among the first to 

 understand, the significance of alternation of 

 generations in such animal groups as coelenterates 

 and parasitic flatworms. B and C, redrawn 

 Irom L. H. Hyman, 7he Invertebrates: Protozoa 

 through Clenophora, copyright 1940 by McGraw- 

 Hill Book Co., Inc., printed by permission.) 



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