144 



CCELENTERA. 



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 circum- 

 ference, 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 



Fig. 42. — Surface view of Amelia. (From Romanes.) 



Showing four genital pockets in centre, much br.inclied radial 

 canals, eight peripheral niches for sense organs, and peripheral 

 tentacles. 



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 (Fig. 43, R). 



To the inside of these genital organs, within the digestive 

 cavity, are four groups of mobile gastric filaments {g-f., Fig. 

 43), which are very characteristic of jelly fish. In appearance 

 these are very similar to the small tentacles of the margin, 

 and, like them, are hollow. They are covered with endo- 

 derm — with ciliated, glandular, muscular, and stinging cells. 



