STRUCTURE OF AURELIA. 133 



a club-shaped projection with numerous calcareous " oto- 

 liths " in its cells, and a couple of apparently sensitive pits 

 or grooves. We are not warranted in calling these organs 

 " optic," " auditory," and " olfactory,'' in Aurelia at any 

 rate. The sense-organs arise as modifications of tentacles, 

 and are often called " tentaculocysts " or " rhopalia." 



Muscular System. — Between the plexus of nerve-cells and 

 the sub-umbrellar mesogloea, there are cross-striped muscle- 

 fibres, each of which has a large portion of non-contractile 

 cell-substance attached to it. They lie in ring-like bundles, 

 and by their contractions the medusa moves. Unstriped 

 muscle-fibres are also found about the tentacles and lips. 



Alimentary System. — The four corners of the mouth are 

 extended as four much frilled "arms," each with a ciliated 

 groove and stinging-cells, and with an axis of mesogloea. 

 They exhibit considerable mobility. Their crumpled and 

 mobile bases surround and almost conceal the mouth. A 

 short tube, the " manubrium " or gullet, connects the 

 mouth with the central digestive cavity which occupies the 

 centre of the disc. From this central chamber, sixteen 

 gastro-vascular canals of approximately equal calibre radiate 

 to the circumference, where they open into a circular canal, 

 with which the hollow tentacles are connected. Eight 

 of the radial canals are straight, but the other eight are 

 branched, and thus in an adult Aurelia the total number of 

 canals is large. These canals are really due to a partial 

 obliteration of the gastric cavity, to a fusion of its ex-umbrellar 

 and sub-umbrellar walls along definite lines. They are all 

 lined by ciliated endoderm. 



Where the manubrium or tube from the mouth passes into 

 the central digestive cavity, there are four strong pillars of 

 thickened sub-umbrellar material. Outside each of these 

 pillars, and still near the base of the manubrium, there are 

 four patches where the sub-umbrellar surface remains thin. 

 These are the gastro-genital membranes, lined internally by 

 germinal epithelium. 



To the inside of these genital organs, within the digestive 

 cavity, are four groups {phacelli) of mobile gastric filaments 

 which are very characteristic of jellyfish. They are covered 

 of course with endoderm — with ciliated, glandular, muscular, 

 and stinging-cells. 



