II. SCYPHOZOA: DISCOMEDUS/E 



223 



septa of the scyphostoma (p. 220). Since the ephync differ markedly 

 from the adult medusas and only gradually change into the sexual form, 

 the alternation of generations is complicated by a metamorphosis. This 

 metamorphosis persists in some cases (Pelagia noctilucd) where the alter- 

 nation of generations is suppressed; the egg develops directly into an 

 ephyra, which transforms into the adult jelly-fish. 



FIG. 188. Development of Aurelia aurita (from Hatschek). First row, growth 

 of planula to scyphostoma; below, strobilation (separation of ephyra?): left, oral view 

 of scyphostoma; right, two ephyra:. 



Order I. Discomedusae. 



The foregoing account applies, as a whole to only the Discomedusa?, the 

 widest distributed and most abundant of the Scyphomedusje. The order is 

 divided into two suborders, I. SEM.*;OSTOME.E, mouth X-shaped with long fringed 

 and very mobile arms at the corners of the mouth. Aurclid flavidula* and 

 Cyanea arctica* common in north Atlantic waters, the latter large, exceptionally 

 seven feet in diameter; Pelagia* Ulmaris (fig. 186). (2) RHIZOSTOMF/K, four 

 oral arms which branch dichotomously; the mouth and grooves on the arms 

 closed by union of their edges so that many small stomata remain through which 

 food is taken. Stomolophus* Polydonia* (fig. 187). 



Certain Scyphomedusas are distinguished from the Discomedusae. Some 

 of these are inhabitants of the deep seas and only recently known; others ditlrr 

 so from the Discomedusae that the relationship was not seen at first. These 

 have in common the rathannua, four partitions, homologous to the ta-niolir of 

 the scyphostoma, which bear the phacellse and divide the peripheral part <>t the 

 gastral cavity in such a way that the gonads are separated into eight groups. 

 The marginal bodies vary in three ways. 



