96 



DISCOPHOR^. 



Part III. 



the direction of the corners of the mouth (o"), and four correspond to the centre 

 of the genital pouches (o'), while the eight broader pouches are immediately above 

 the eight Ijunches of tentacles. 



With these data, Ave may now pi'oceed to an inquiry into the combination of 

 these structural elements, with reference to their homology with similar parts in 

 Aurelia, and, at the same time, with reference to the whole constitution of a Cyanea. 

 We have seen, that in Aurelia there are four simpler systems of radiating tubes, 

 alternating with the genital pouches, the main branch of which terminates at four 

 eyes, and which we were led to consider as the ambulacral system of that genus. 

 This conclusion was founded upon the fact, that these systems correspond to the 

 corners of the mouth and alternate with the sexual pouches. For the same reasons, 

 we shall consider the four narrow pouches of the Cyanea, which are in the same 

 trend with the corners of the mouth, and also alternate with the genital pouches, 

 as the four ambulacral systems of Cyanea; and the four other narrow pouches, 

 in the radial j^i'olongation of the genital pouches, as the interambulacral system, 

 since they stand in the same relation to the genital organs as the more compli- 

 cated system of radiating tubes in Aurelia, the branches of which arise from the 

 genital pouches. The only question Avhich may here present some difficulty is 

 the connection of the broader pouches, of which we have seen that there are eight 

 in Cyanea. At first sight, it might appear as if there was nothing like them in 

 Aurelia, and as if they, alone, should be considered as interambulacral structures.^ 

 Their alternate position, between the narrow i^ouches corresponding to the corners 



^ At the time of the publication of the third 

 volume of this work, I was still under the impres- 

 sion that the eight broad pouches alone belong to 

 the interambulacral system, and considered then the 

 four narrow pouches, in the direct prolongation of 

 the genital pouches, as ambulacral. I had not yet 

 divested myself of the belief that the presence of 

 an eye, at the termination of these pouches, indi- 

 cates an ambulacral structure. A closer comparison 

 of Cyanea with Aurelia has satisfied me that I was 

 mistaken. There can be no doubt, in Aurelia, tliat 

 the complicated system of radiating tubes is, in its 

 totality, the peripheric prolongation of the genital 

 pouches, and therefore entirely interambulacral. It 

 is equally certain now, that the broad pouches of 

 Cyanea are homologous to those simple chymifer- 

 ous tubes of the Aurelia which terminate at the 

 margin without eyes; they must, therefore, by ho- 



mology, be considered as the lateral parts of the 

 nari'ow pouches in the direct prolongation of the 

 genital pouches, with which they freely communi- 

 cate, and on tjiat account be referred, two and two, 

 as part of that interambulacrum, to which the nar- 

 row pouch which they embrace belongs. In the 

 chapter on Aurelia, I have already alluded to Tia- 

 ropsis, as furnishing satisfactory evidence that the 

 presence of eyes does not necessarily indicate an 

 ambulacral structure, since this genus has no eyes 

 in the prolongation of the radiating chymiferous 

 tubes, while there are two in each interambulacrum. 

 Moreover, the ambulacra of a large number of Ra- 

 diates terminate without eyes, as, for instance, in 

 all Holothurians, in all Crinoids, and in all Ophi- 

 urans, while they are well developed in all Asteri- 

 ans and in all Echinoids. They are also wanting 

 in most Poly[)S. 



