THE SCYPHOMEDUSAE 67 



ORDER 4. Discomedusae. 



DEFINITION. Scyphomedusae with four perradial and four 

 interradial (sometimes more) tentaculocysts. The radial canals 

 are either broad pouches or fine canals, and are often very 



Fiu. 0. 



Later development of Chrysaora and Aurelia (after Claus). A, scyphistoma of Chrysaora, 

 with four perradial tentacles and homy basal perisarc. B, oral surface of later stage of 

 scyphistoma of Aurelia, with comnieiiceJiient of four interradial tentacles. The quadrangular 

 mouth is seen in the centre ; the outline of the stomach wall, seen by transparency around it, 

 is nipped in four places interradially to form the four gastric ridges. C, oral surface of a sixteen- 

 tentacled scyphistoma of Aurelia. The four gastric interradial ridges are seen through the 

 mouth. D, lirst constriction of the Aurelia scyphistoma to form the pile of ephyrae or young 

 medusae (see Fig. 7). The single ephyra carries the sixteen scyphistoma tentacles, which will 

 atrophy and disappear. The four longitudinal gastric ridges are seen by transparency. E, young 

 pphyra just liberated, showing the eight bifurcate arms of the disc and the interradial single 

 gastral filaments. F, ephyra developing into a medusa by the growth of the adradial regions. 

 The gastral filaments have increased to three in each of the four sets. A, margin of the mouth ; 

 Ad, adradial radius ; F , gastral lilament ; In, interradial radius ; JG, adradrial gastral canal ; 

 JR = R*, adradial lobe of the disc ; K, lappet of a perradial arm ; M, stomach wall ; Mat, muscle 

 of the mesentery ; Jl/jp, mesentery ; Ms, mesoderm ; 0, tentaculocyst ; P, perradial radius ; 

 HP, interradial radius ; I&, adradial radius ; SG, commencement of circular canal. 



numerous they are not separated from each other by laminar 

 mesenteries, and a well-marked endoderm lamella unites them. 

 Development showing alternation of an asexual scyphistomoid 

 with a sexual medusoid generation. 



The Discomedusae are probably all a metagenetic hydroid- 

 like form alternating with a sexual medusoid generation. In 



