1'HYU M OKLKNTKRATA 



183 



RhizofltomcR have been found wit li prey of consideralile size, such as fishes, em- 

 braced by the arms and partly drawn into these apertures, whiqh are therefore 

 called the mttoricd mouth*. 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 

 (at.). We thus get a, polystomn/oit* or many-mouthed condition which is practi- 

 cally 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 



rod.c 



STTltil 



Fie;. 135. — Pilema pulmo. A, side view of the entire animal ; B, vertical section, diagrammatic ; 

 C, one of the suctorial mouths, magnified, c. arm canal ; .</. /. gastric filaments ; yon. gonads ; 

 or. 'i. oral arms ; nul. c. radial canal ; s. mth. suctorial mouths ; at. stomach ; tl, t2, tS, tentacles 

 on oral arms. (After Cuvier, Claus, and Huxley.) 



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 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 Rhizostomaj 

 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 sub-genital portico, which lies immediately below the 

 floor of the stomach and above the brachial disc. 



