TYPE CCELBNTERA. ■ 99 



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

 ccelenteric cavity a number of coarse thread-like iilaments, 

 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 



o 



Pig. 51. — Marginal Sense-organ op Rhopalonema (after Hertwig). 

 ce = ccelenteric cavity. en = endoderm. 



cp = covering plate. o = eye. 



ec = ectoderm. oi = 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 



