IV 



PHYLUM CCELENTERATA 



181 



Cubomedusse are the only Scyphozoa which, like the Hydrozoa, have a com- 

 plete nerve-ring. The tentaculocysts are very complex, each bearing a litho- 

 c}'st 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 

 orc l er 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 primarily 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 Seniostomse and 



st 



B 



FIG. 136. Filema pulmo. A, side view of the entire animal ; B, vertical section, diagram- 

 matic ; C, one of the suctorial mouths, magnified, c. arm canal ; g.f. gastric filaments ; 

 gon. gonads ; or. a. oral arms ; rad. c. radial canal ; s. mth. suctorial mouths ; st. stomach ; 

 tl, t2, (3, tentacles on oral arms. (After Cuvier, Claus, and Huxley.) 



Rhizostomse are large, and one of the former group Cyanea arclica 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 hollow and often of great length in the Semo- 

 stomce (Fig. 127), and altogether absent in the Rhizostomse (Fig. 136). In the 

 Semostomae there are four oral arms (Fig. 127, or. a.), each resembling a leaf 

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

 stomse each of the original four arms (Fig. 136, or. a.) becomes divided longi- 

 tudinally 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. 



