Chap. H. HOMOLOGICAL RELATIONS. 67 



these sexual chymifei'ous systems, cannot be an objection to considering these systems 

 as interambulacral structures, since we have already seen that in Tiaropsis the eyes 

 are not in the ambulacral rays, but in the interambulacral spaces; and the presence 

 of chymiferous tubes in the interambulacral spaces is no more exceptional in these 

 Medusae, than in many Echinoderms, among which I have observed and described 

 them in Echinarachnius, more than twelve years ago.^ An objection to this expla- 

 nation might perhaps be made on the ground that, in so viewing the Discophorae, 

 the parts considered as interambulacral are more extensive, more conspicuous, and 

 more characteristic than those regarded as ambulacral. No doubt they are ; but this 

 does not alter their homologies, any more than the fact that in Cidaris the ambulacra 

 are also much narrower, and less conspicuous than the interambulacra. Indeed, the 

 relative development of the ambulacral and interambulacral zones varies from one 

 family to the othei', in one and the same class, throughout the type of Radiates. 

 A more direct comparison of Aurelia {Fiff. 2) and Echinarachnius {Fi(/. 3), or 

 some other member of the family of the Scutellidas, 

 cannot fail to remove other doubts, respecting the close '^' 



structural resemblance of the Acalephs and Echinoderms, 

 which may linger in the minds of those who have be- 

 come accustomed to consider them as belonging to differ- 

 ent types. In the first place, the j^revailing idea that 

 while Acalephs have a body consisting of a continuous 

 mass of gelatinous substance, in which there are only 

 limited cavities, the Echinoderms have thin, solid walls, 

 surrounding a wide hollow space, in which all the organs EcHixAKAci.K.ts parma. 



.- ' 1 1 ' n n ° oral aperture.— c e e ambulacra. — c and 



oi the body are mclosedj is far from accurate. In many "' ambuiacrai ramification?, -wio inter- 



_ II* T 1 ambulacra. 



of the bcutellida3, the central cavity of the body is hardly 



more extensive than that of Aurelia, and certainly not so wide as that of Cyanea; 

 and far from being circumscribed by thin walls, it is surrounded by a spongy mass 

 quite as continuous, and forming as large a proportion of the bulk of the body, as 

 the disk of any Medusa. The difference in the rigidity of that mass cannot be 

 considered as typical, any more than the peculiarity of the skeleton of the Selachians 

 or Myzonts constitutes a typical difference between them and the other Vertebrates. 

 Moreover, among the Echinoderms there are those, such as the Holothurians, the body 

 walls of which are not rigid ; and among the Acalephs there is a numerous group, 

 the Tabulata, the largest part of the body of which is as rigid as the hard-shell 

 Echinoderms. All this goes to prove, that among the Radiates, the distinctions 

 adopted upon the ground of the presence or absence of solid parts, are losing their 



^ Comptes-Rendus de rAcademie des Sciences for 1847, in a letter to Humboldt, p. 677. 



