CHAPTER V. 



THE SCYPHOMEDUSAE. 1 



CLASS SCYPHOMEDUSAE. 



Order 1. Stauromedusae. 

 2. Peromedusae. 

 3. Cubomedusae. 

 ,, 4. Discomedusae. 



Sub-Order 1. Cannostomae. 

 ,, 2. Semostomae. 



,, 3. Rliizostomae. 



DEFINITION. Coelenterata which typically present two main 

 forms of individuals the non-sexual scyphistoma (hydroid) and 

 a the sexual medusoid ; in this case the life- 



history presents an alternation of generations 

 in which the scyphistoma produces the medu- 

 soid by transverse strobilation, and the sexual 

 cells of the medusoid develop into a scyphi- 

 stoma. In other cases the medusoid may 

 develop directly from the sexual cells. Gastric 

 ridges (taeniolae or mesenteries) occur in both 

 scyphistoma and medusoid, gastric filaments 

 (phacellae) in the medusoid. The sexual cells 

 lie typically in interradii, and are developed 

 from endoderm. The medusoids are devoid of 

 a velum; a velarium is sometimes present; the 

 sense organs are tentaculocysts and cordyli. 



In the Scyphomedusae, as in the Hydro- 

 medusae, but by a different path, the seg- 

 mentation of the fertilised ovum produces a 

 larva of the diblastula type (cf. p. 2), the endo- 

 derm of which is formed by invagination, and not by delamination 

 from the ectoderm. From this diblastula may grow either of two 

 forms of individual the hydroid or the medusoid. 



1 By G. Herbert Fowler, B.A., Ph.D. 



FIG. 1. 



Longitudinal section of 

 a diblastula (gastrula), 

 formed by invagination of a 

 simple blastula at one pole, 

 a, orifice of invagination 

 (blastopore) ; 7>, coelen- 

 teron ; c, endoderm ; cf, 

 ectoderm. (After Gegen- 

 baur, from Lankester.) 



