The Medusoid Stage of a Scyphozoan. 23 



THE MEDUSOID STAGE OF A SCYPHO- 

 ZOAN 



{Cyanea arc tic a). 



External Anatomy. - - In many points of its general 

 anatomy the present form agrees with the medusa of the 

 Campanularian Hydroid. A rounded aboral, abac final, or 

 exumbrella surface is distinguishable from an oral, actinal, 

 or subumbrella surface. The former is generally convex, 

 while the latter is more often concave. The margin is 

 divided into eight large lobes. 



The mouth will be found in' the center of the subumbrella, 

 but surrounded and probably hidden by a mass of delicate 

 fringe-like tissue, which, in life, may hang from the lower 

 surface of the animal in four pleated folds. These folds, 

 which are known as the oral tentacles, are capable of con- 

 siderable extension and contraction, and direct the food to 

 the mouth. 



The oral tentacles may now be cut away, leaving only 

 their line of attachment along the margin of the oral open- 

 ing or mouth. This opening is elevated upon a short manu- 

 brium, and has four radiating angles which determine four 

 main radii of the animal, the so-called perradii. The four 

 radii that might be drawn between the perradii are known 

 as interradii. The perradii and interradii extended to the 

 margin pass directly through eight " marginal sense-organs" 



