IV 



PHYLUM CCELENTERATA 171 



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 

 the veins of a leaf, towards the edge of the arm. As development proceeds, 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 Toy 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. 



In Ephyra, the lowest of the Cannostomse, only four gastric filaments are 

 present, 'as in Tessera (p. 165) or a*newly liberated Ephyrula (p. 163), but as a 

 rule these characteristic structures are very numerous. The lower forms, also, have 

 no sub-genital pouches, or indeed anything corresponding to the septal funnels of 

 the preceding orders. In the higher Rhizostomae a remarkable modification is 

 produced in connection with these cavities : the four pouches approach the centre 

 and fuse with one another, forming a single spacious chamber, the sub-genital 



m 



FIG. 123. Pelagia noctiluca : Three developmental stages, m. mouth ; /. marginal lappet ; 

 s. tentaculocyst. (From Korschelt and Heider, after Kro'hn.) 



portico, which lies immediately below the floor of the stomach and above J}he 

 brachial disc. ' 



In many of the Discomedusae 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 medusae. But in 

 other cases there is no alternation of generations, and development is direct. For 

 instance, in Pelagia (Fig. 123) one of the Semostomae a blastula is formed 

 which becomes invaginated at one end, forming a gastrula. The blastopore or 

 gastrula-mouth remains open, and a considerable space is left between the 

 invaginated endoderm and the ectoderm. Next the mouth region becomes 

 elevated, forming a manubriimi, and around, this a circular depression appears - 

 the rudiment of the sub-umbrellar cavity surrounded .by a raised ridge, the 

 umbrella margin, which soon becomes divided into lobes, the marginal lappets. 

 Up to this tin\e the embryo- is ciliated externally, but soon the cilia disappear, 

 and the little creatures assume somewhat the form of an Ephyrula, which 

 gradually develops into the adult Pelagi-a. 



ADDITIONAL REMARKS ON THE SCYPHOZOA. 



The Scyphozoa are all marine, and the majority are pelagic, i.e. 

 -swim freely on the surface of the ocean. A few inhabit the deep 

 sea, and have been dredged from as great a depth as 2,000 fathoms. 



