182 



ZOOLOGY 



SECT. 



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

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

 septa (mesenteries). In the Semostomse (Fig. 127) 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 Rhizostomse (Fig. 1 36, 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. Rhizostomes have been found with prey of considerable size, 

 such as fishes, embraced by the arms and partly drawn into these apertures, 

 which are therefore called the suctorial mouths. They lead into canals in the 

 thickness of the arms (B, c.), the lesser canals unite into larger, and then finally 

 open into the stomach (st.). We thus get a polyslomatous or many-mouthed 

 condition which is practically unique in the animal kingdom, the only parallel 

 to it being furnished by the Sponges, in which the inhalant pores are roughly 

 comparable with the suctorial mouths of a Rhizostome. 



It has been found that this characteristic arrangement is brought about by 

 certain changes taking place during growth. The young Rhizostome has a 



single mouth in 

 the usual position, 

 and more or less 

 leaf - like arms, 

 folded along the 

 midrib so as to 

 enclose a deep 

 groove, from 

 which secondary 

 grooves pass, like 

 y, the veins of a leaf, 

 Tf ^ s towards the edge 



of the arm. As 



Three developmental stages, m. j^^i 

 s. tentaculocyst. (Fom Korschelt development pro- 



ceeds, these 

 grooves become 



converted into canals by the union of their edges, thus forming a system of 

 branching tubes opening proximally into the angles of the mouth and distally 

 by small apertures -the suctorial mouths- on the edges of the arms. At 

 the same time the proximal ends of the arms grow towards one another and 

 finally unite across the mouth, closing it completely, and forming a strong 

 horizontal brachial disc, which in the adult occupies the centre of the sub- 

 umbrellar surface. 



The gastric filaments are usually very numerous. In the higher Rhizo- 

 stomse a remarkable modification is produced in connection with the sub-genital 

 pouches ; the four pouches approach the centre and fuse with one another, 

 forming a single spacious chamber, the suh-gewiial portico, which lies imme- 

 diately below the floor of the stomach and above the brachial disc. 



In many of the Discomedusse development takes place in the same general 

 way as in Aurelia, i.e. the impregnated egg gives rise to a scyphula or asexual 

 polype stage, which, by transverse division, produces sexual medusse. In 

 Cassiopeia the scyphula arising from the fertilised ovum gives off buds which 

 become detached as free-swimming planulae, and these, coming to rest, develop 

 into soyphulse. But in other cases there is no alternation of generations, and 

 development is direct. For instance, in PeJagia (Fig. 137) one of the 

 Semostonife a blastula is formed which becomes invaginatecl at one end 



FIQ. 137 

 mouth 



Felagia noctiluca : 



r. marginal lappet : 



and Heider, after Krohn.) 



