74 



THE SCYPHOMEDUSAE 



pouches run the radial canals, the arrangement of which falls 

 under two main types. In the one type sixteen very broad and 

 shallow pouches (perradial, interradial, and adradial) pass to the 

 edge of the bell and end blindly (Pelagia) ; each may bifurcate, 

 and may give off short caeca, which never anastomose. In the 

 second type narrow canals are formed, primarily to the number of 

 sixteen, which may remain simple (Floscula) or branch (Aurelia) 

 and anastomose (Leptobrachia) ; the number of canals may 



FIG. 14. a, Ehizostoma pulmo ; 6, Chrysaora hyosceUa. (From Lankester.) 



amount to thirty-two or sixty-four. In this second type the 

 radial canals open into a circular canal. As in the Hydromedusae, 

 the whole system of radial canals and pouches is produced by a 

 concrescence of exumbral and subumbral endoderm, traces of which 

 generally persist throughout life as an endoderm lamella. 



The phacellae are of the usual character, and interradially 

 placed ; they may be only four in number, but are generally very 

 numerous. 



The generative organs are typically four in number and inter- 

 radial in position, and are formed from the subumbral endoderm 

 either of the gastric cavity or of the radial pouches. In some 

 Cannostomae they become secondarily divided so as to form eight 

 adradial organs (Nausicaa). They are primitively horseshoe-shaped 

 thickenings of the endoderm, either convex centrally (Palephyra), 

 or concave centrally (Aurelia) ; they may become folded (Pelagia), 

 or thrown into lappets (Chrysaora), and may either be evaginated 



