TYPE CCELENTERA. 99 



Finally, iu the line of each interradius there project into the 

 ccelenteric cavity a number of coarse thread-like filaments, 

 the mesenterial filaments (mf\ which are unrepresented in the 

 Hydromedusse. 



Such is the general structure of the Scyphomedusse ; the 

 modifications will be better described in connection with the 

 various orders into which the class may be divided. In histo- 

 logical structure the resemblance to the Hydromedusse is so 

 great as to do away with the necessity of a detailed account, 

 except as regards the sense-organs. As already stated, these 

 when present are modified tentacles and partake of the char- 

 acters of both eyes and otocysts. They are usually short 

 finger-like stalks, lying in a notch of the rim of the bell 

 and covered over by folds (covering plates, Fig. 51, cp) arising 

 from the adjacent substance of the bell on either side of the 



ec 



o 



FIG. 51. MARGINAL SENSE-ORGAN OF Rhopalonema Cafter HERTWIG). 

 cc = ccelenteric cavity. en = endoderm. 



cp = coveriug plate. o eye. 



ec = ectoderm. ot = otocyst. 



niche and frequently uniting so that the stalks lie in pouch-- 

 like cavities. The ectoderm of the finger-like stalks contains 

 numerous sensory and ganglion cells, and at one or more 

 regions pigment-cells are associated with these to form the 

 eye (o), which may be further perfected by the addition of a 

 cuticular lens. The stalks are hollow, containing a prolonga- 

 tion of the ccelenteric cavity (cc) lined by endoderm, and at the 

 tip of the stalk the endoderm-cells are filled with crystals of 

 carbonate of lime, the whole mass of crystals forming a rather 

 large otocyst (ot). The covering plates, furthermore, above the 

 sensory stalks are usually grooved, the bottom of the groove 



