164 ZOOLOGY SECT. 



The Scyphozoa are divisible into four orders, as follows : 



ORDER 1. STAUROMEDUS^E. 



Scyphozoa having a conical or vase-shaped umbrella, sometimes 

 attached to external objects by an ex-umbrellar peduncle : no 

 tentaculocysts. 



ORDER 2. PEROMEDUS.E. 



Scyphozoa having a conical umbrella divided by a transverse 

 constriction : four inter-radial tentaculocysts. 



ORDER 3. CUBOMEDUS^E. 



Scyphozoa with a four-sided cup-shaped umbrella: jt>ur per- 

 radial tentaculocysts. 



ORDER 4. DISCOMEDUS.E, 



Scyphozoa with a flattened saucer- or disc-shaped umbrella : 

 not fewer than eight tentaculocysts, four per- and four inter-radial. 



Sub-Order a. Cannostomce. 



Discomedusae with a simple square mouth devoid of oral arms. 



Sub-Order 1. Semostomce. 



Discomedusse in which the square mouth is produced into four long oral 

 arms. 



Sub-Order c. Rliizostomw. 



Discomedusse having the mouth obliterated by the growth across it of the 

 oral arms : the stomach is continued into canals which open by funnel-shaped 

 apertures on the edges of the arms. 



Systematic Position of the Example. 



Aurelia aurita is one of several species of the genus ^Aurelia, 

 and is placed in the family Ulmaridce, the sub-order Semostomce, 

 and the order Discomedusce. 



Its saucer-shaped umbrella and eight tentaculocysts place it at 

 once among the Discomedusse : the presence of a distinct mouth 

 surrounded by four oral arms excludes it from the first and third 

 sub-orders of Discomedusae and place it in the second sub-order or 

 Semostomse. The latter group contains sixiamilies, characterised 

 mainly by differences in the canal -system : the Ulmaridse are 

 distinguished by narrow branched radial canals opening into a 

 circular canal. Of the seven genera in this family, Aurelia stands 

 alone in having its tentacles attached on the dorsal or ex-umbrellar 

 side of the margin, and in the oral arms showing no trace of bi- 

 furcation. Eight species of Aurelia are recognised, A. aurita 

 being distinguished by having the oral arms slightly shorter 



