148 



PHYLUM C(KLENTEKA. 



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 found about the tentacles and 

 lips. 



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 mesogloea. 



They exhibit considerable 

 mobility. Their crumpled 

 and mobile bases surround 

 and almost conceal the 

 mouth. A short gullet or 

 j| " manubrium ' connects 

 the mouth with the diges- 

 tive cavity in the centre of 

 the disc. From this central 

 chamber sixteen gastro- 

 vascular canals of approxi- 

 mately equal calibre radiate 

 to the circumference, where 

 they open into a circular 

 canal, with which the hol- 



Showing four genital pockets in centre, loW tentacles are Connected, 

 much branched radial canals, eight peri- Eight of the radial Canals 

 uheral niches for sense organs, and peri- i , i .-, . , 



pheS tentacles. 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. 68, R.\ 



To the inside of these genital organs, within the digestive 



FIG. 67.- 



-Surface view of Aurelia, 

 From Romanes. 



