TYPES OF CCELENTERA AUREL1A AURITA. 139 



digestive cavity, is continued out into its radiating canals, 

 and is ciliated throughout. The mesoglcea is a gelatinous 

 coagulation containing wandering amoeboid cells from the 

 endoderm. The whole animal is very watery ; indeed, the 

 solid parts amount to not more than ten per cent, of the 

 total weight. 



Nervous system. — The nervous system consists — (a) of a 

 special area of nervous epithelium, associated with each of 

 the eight sense organs, and (l>) of numerous much-elongated 

 bipolar ganglion cells lying beneath the epithelium on the 

 under surface of the disc. This condition should be con- 

 trasted with that in Craspedote medusoids, but too much 

 must not be made of the contrast, for a nerve-ring is 

 described in Cubomedusse, one of the orders of Acraspe- 

 dote jelly-fish. In Aurelia the sense organs are less 

 differentiated than in many other jelly-fish. Each of the 

 eight organs, protected in a marginal niche, consists of a 

 pigmented spot, a club-shaped projection with numerous 

 calcareous " otoliths " in its cells, and a couple of apparently 

 sensitive pits or grooves. The sense organs arise as modi- 

 fications of tentacles, and are often called " tentaculocysts " 

 or "rhopalia." Their cavities are in free communication 

 with branches of the radial canals. 



Muscular system. — Between the plexus of nerve cells 

 and the sub-umbrellar mesoglcea 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 mesoglcea. 

 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 



