182 ZOOLOGY SECT. 



supporting layer of mesogkea. Such a false velum, like the produced thin edge 

 of the umbrella in Aurelia, is known as a velarium. 



The mouth is situated at the end of a short manubdum (mnb.) leading into a 

 wide stomach, from which go off four very broad shallow per-radial pouches 

 (rad. p.), occupying the whole of the four flat sides of the umbrella, and 

 separated from one another by narrow inter-radial septa or partitions (mesenteries) 

 placed at the four corners. These pouches are equivalent to wide radial canals, 

 and the partitions between them to a poorly developed endoderm lamella (end. 

 lam.). At the margin of the umbrella the pouches communicate with one 

 another by apertures in the septa, so that a kind of circular canal is produced 

 (circ. c.), which is divided into chambers by the mesenteries. Near the junction 

 of the gastric pouches with the stomach are the usual four groups of gastric 

 filaments (g. /.). 



The gonads (yon.) are four pairs of narrow plate-like organs, attached one 

 along each side of each inter-radial septum. The nervous system takes the form 

 of a sinuous nerve-ring round the margin of the bell, bearing a distinct group of 

 nerve-cells at the base of each tentaculocyst and tentacle. The Cubomedusa; are 

 the only Scyphozoa which, like the Hydrozoa, have a complete nerve-ring. The 

 tentaculocysts are very complex, each bearing a lithocyst and several eye-spots. 



ORDER 4. DISCOMEDUS.E. 



The preceding orders are all small ones, i.e. , include a small number of genera 

 and species. The vast majority of Scyphozoa belong to the present order the 

 " Disc- jellies " or " Sea-blubbers " as ordinarily understood. 



The umbrella is always comparatively flat, having the form of an inverted 

 saucer. The edge is produced primitively into eight pairs of marginal lappets, 

 but in some of the more highly differentiated forms the number both of lappets 

 and of tentaculocysts becomes greatly increased. Most of the Semostomae and 

 Rhizostomae are large, and one of the former group Cyanea arctica may attain 

 a diameter of 2 metres and upwards, while its marginal tentacles reach the 

 astonishing length of 40 metres or about 130 feet. But in spite of their size and 

 apparent solidity, the amount of solid matter in these great Jelly-fishes is extra- 

 ordinarily small ; some of them have been proved to contain more than 99 per 

 cent, of sea-water. 



The marginal tentacles are hollow and often of great length in the Semostomae 

 (Fig. 126), and altogether absent in the Rhizostomse (Fig. 135). In the 

 Semostomae there are four oral arms (Fig. 126, o r. a.), each resembling a leaf 

 folded along its midrib, and having more or less frilled edges : in the Rhizostomae 

 each of the original four arms (Fig. 135, or. a.) becomes divided longitudinal!}- in 

 the course of development, the adult members of the group being characterised 

 by the presence of eight arms, often of great length, and variously lobed and 

 folded so as to present a more or less root-like appearance. 



The arrangement of the enteric cavity and its offshoots presents an interest- 

 ing series of modifications. In no case are there any tamioles or inter-radial 

 septa (mesenteries). In the Semostomae (Fig. 126) the stomach-lobes give off 

 well-defined radial canals, which are frequently more or less branched, often 

 unite into complex networks, and sometimes open into a circular canal round the 

 margin of the umbrella. 



In the Rhizostomae (Fig. 135, B) a similar network of canals is found in the 

 umbrella, but an extraordinary change has befallen the oral or ingestive portion 

 of the enteric system. Looking at the oral or lower surface of one of these Jelly- 

 fishes, such as Pilema, no mouth is to be seen, but a careful examination of the 

 oral arms shows the presence of large numbers hundreds, or even thousands in 

 some cases of small funnel-like apertures (B, C, s.mth.) with frilled margins. 



