PHYLUM CCELENTERATA 



1G9 



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 Cubomedusse 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. Many of the species belonging 



FIG. 121. Nausithoe. The entire animal from the oral aspect, ar. adradii ; g. goiiads ; g.f. 

 gastric filaments ; ir. inter-radii ; 7/. circular muscle of sub-umbrella ; pr. per-radii ; rl. tenta- 

 culocysts ; sr. sub-radii ; t. tentacles. The black cross in the centre represents the mouth. 

 (From Lang's Comparative Anatomy.) 



to the lowest sub-order the Cannostomse are small, not exceeding a few 

 millimetres in diameter, but most of the Semostomse 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 extraordinarily small ; 

 some of them have been proved to contain more than 99 per cent, of sea-water. 



The marginal tentacles are short and solid in the CannostomaB (Fig. 121), 

 hollow and often of great length in the Semostomog (Fig. 113), and altogether 

 absent in the Rhizostomoe (Fig. 122). The oral arms are absent in the Canno- 

 stoimv (Fig. 121), where there is a single square or four-rayed mouth : in the 

 Semostomiv there are four oral arms (Fig. 113, or. a.), each resembling a leaf 



