154 PHYLUM CCELENTERA. 



Alimentary system. The four corners of the mouth are 

 extended as four much-frilled lips, each with a ciliated 

 groove and stinging cells, and with an axis of mesoglcea. 

 They exhibit considerable mobility. Their crumpled and 

 mobile bases surround and almost conceal the mouth. A 

 short gullet or " manubrium " connects the mouth with the 

 digestive cavity in 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 by a fusion of 

 its ex-umbrellar and sub-umbrellar walls along definite lines. 

 They are all lined by ciliated endoderm. 



Where the gullet passes into the central digestive cavity, 

 there are four strong pillars of thickened sub-umbrellar 

 material. Beside these pillars, there are four patches 

 where the sub-umbrellar surface remains thin. These are 

 the gastro-genital membranes, lined internally by germinal 

 epithelium (Fig. 74, R.}. 



To the inside of these genital organs, within the digestive 

 cavity, are four groups of mobile gastric filaments (g.f. y Fig. 

 74), which are very characteristic of jelly-fish. In appear- 

 ance these are very similar to the small tentacles of the 

 margin, and, like them, are hollow. They are covered with 

 endoderm with ciliated, glandular, muscular, and stinging 

 cells. 



The body is mapped out into regions by the following convention : 

 The first tentacles to appear in the larva are four in number, and 

 correspond to the four angles of the mouth ; the radii on which they 

 appear are called "perradial," marked by the four lips. Half-way 

 between these, four " interradials " are then developed, marked by the 

 gonads and gastric filaments. Then eight "adradials" may follow, 

 between perradii and interradii, marked by the eight unbranched 

 radial canals. 



Reproductive system. The sexes are separate. The. 

 reproductive organs ovaries or testes consist of plaited 

 ridges of germinal epithelium, situated on the four patches 

 already mentioned, within sacs which are derived from and 



