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: our 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. 



Discomedusce with a simple square mouth devoid of oral arms. 



Sub-Order b. Scmostomce. 



Discomedusa? in which the square mouth is produced into four long oral 

 arms. 



Sub- Order c. Rliizostorn a\ 



Discomedusae 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 Ulmarida', the sub-order Semostomcc, 

 and the order Discomedusce. 



Its saucer-shaped umbrella and eight tentaculocysts place it at 

 once among the DiscomedusaB : the presence of a distinct mouth 

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

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

 Semostoma?. The latter group contains six families, characterised 

 mainly by differences in the canal-system : the Ulmarida? 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 



