Chap. III. ACTINAL SYSTEM OF CYANEA. 97 



of the mouth and those in the direct prolongation of the genital pouches, fevors 

 the view already discussed, of the possibility of eight spheromeres in Aurelia. But 

 a more careful comparison between Cyanea and Aurelia, will disclose an unexpected 

 correspondence between the two, in the relation to the broad pouches flanking the 

 narrow pouches in the prolongation of the genital pouches and the complicated 

 system of radiating tubes arising from the genital pouches in Aurelia. In the 

 latter genus this system exhibits three main branches, starting from each pouch, 

 the middle of which terminates at an eye, while the outer ones, which unquestion- 

 ably arise from the same genital pouch, border on the simpler ambulacral systems 

 of radiating tubes. Now, in Cyanea, the narrow pouches, in the direct prolongation 

 of the genital pouches, which terminate at an eye, also correspond to the middle 

 main branch of the complicated system of Aurelia, while the broader pouches, on 

 each side of these, correspond to the outer main branches of the complicated 

 system of Aurelia. To complete the identity, it may easily be ascertained that 

 the broader pouches communicate freely with the genital pouches, as seen PI. IV. 

 Fi(/. 1, in the prolongation of d", and in the prolongation of a and a', where 

 the sack of the genital poviches has been removed. There can be no doubt, there- 

 fore, that, widely developed and highly complicated as these systems may appear, 

 the whole segments, from which hang a genital pouch and two adjoining bundles 

 of tentacles with their broad pouches, as well as the narrow pouch between them, 

 terminating in an eye, are interambulacral systems. To facilitate comparisons in 

 tracing these homologies, the figures representing our Cyanea on Pis. IV. and V., 

 have been drawn exactly in the same position as those of Aurelia, in Pis. VI. 

 and VII. With these facts before us, it must be evident that the indentations 

 along the margin of Cyanea, though a very important feature in the form of 

 that genus, in no way indicate the organic divisions of its body. Nor are, indeed, 

 these indentations homologous to those of Aurelia, according to the degree of their 

 prominence; for in Aurelia the most marked indentations correspond to the position 

 of the eyes, and the least marked ones, to the termination of the simple branches 

 of the complicated system of radiating tubes, which have no eye; while in Cyanea 

 the lesser indentations are- in front of the eyes, and the deeper indentations in 

 front of the wider pouches, which correspond, as we have just seen, to the simple 

 radiating tubes without eyes. 



The essential elements of the structure of Cyanea, therefore, are four narrow 

 ambulacra, in the direction of the four corners of the mouth, alternating with four 

 very wide and complicated interambulacra, facing the genital pouches. In these 

 interambulacra we may distinguish the middle pouch, which is in the direction of 

 the centre of the genital pouches, and the tentacular or lateral pouches, of which 

 there is one on each side of the middle pouch. The middle pouch, in its peri- 



TOL. IV. 13 



