AURELIA AURITA 37 



The medusa may be divided into eight symmetric sectors 

 or octants by four planes intersecting each other at 45 in the 

 longitudinal axis. For the sake of convenience two of the planes 

 intersecting at right angles are called perradial, and the other 

 two interradial. The planes which divide the disc into sixteen 

 sectors are called adradial. Neither of the adradial planes di- 

 vides the medusa into symmetric halves. The various organs 

 are said to be perradial, interradial or adradial in accordance 

 with the name of the plane which bisects them. 



The margin of the medusa is not an unbroken circle. There 

 are eight notches in the margin corresponding to the four planes 

 of symmetry. At the bottom of each notch is a sense organ or 

 rhopalhim. Four of them are perradial and four interradial. 

 On each side of a rhopalium is a large marginal lappet. The 

 margin of the medusa is covered with numerous small tentacles. 

 These tentacles arise from the aboral surface at a very short 

 distance from the margin and alternate with small marginal 

 lobules. The tentacles are hollow, distended at their base; their 

 cavity is in direct communication with the circular canal of the 

 gastro-vascular system (ccelenteron) and is lined with endoderm. 

 Each tentacle has on its subumbral side longitudinal muscle 

 fibers in the subepithelial layer of the ectoderm and a row of 

 broken rings of nematocysts on its exumbral side. 



Body covering and muscular system. The body covering 

 consists of a single layer of ectoderm. On the exumbrella the 

 cells of the covering belong to the type of very flat epithelial 

 cells with numerous nematocysts arranged in batteries on little 

 protuberances, especially prominent toward the edge of the disc. 

 On the subumbrella the number of nematocysts is small and 

 they are irregularly scattered among the common epithelial 

 cells. The subepithelial layer is a well developed system of 

 circular and radial muscular fibres. The circular fibres are 

 especially numerous toward the edge of the disc. The radial 

 fibres run from the centre toward the tentacles. The contraction 

 of the disc is accomplished by the joint action of the circular and 



