64 DISC0PHOR.E. • Part III. 



SECTION IV. 



HOMOLOGICAL RELATIONS OF AURELIA AND ECHINODERMS. 



Leuckart, and with him most of the German naturalists, have urged their convic- 

 tions of a typical difference between the Acalephs and Echinoderms with so much 

 confidence, that, holding, as I do, the contrary opinion, I feel bound to avail myself 

 of every opportunity of opposing their conclusions; and Aurelia furnishes so striking 

 an instance of a close resemblance to Echinarachnius, that, as a complement to the 

 anatomical description of our Medusa, I may be permitted to compare, more closely 

 than might otherwise be necessary, two representatives of the classes in question. 

 That the plan of structure of the Coelenterata bears a striking resemblance to that 

 of the Echinodermata, is, I believe, conceded even by those who would separate 

 them, as two primaiy divisions of the animal kingdom. But it is not generally 

 luiderstood that this resemblance is founded upon as perfect an identity of the 

 structural elements of the two divisions as exists between the classes of Vertebrata; 

 for were this identity fully appreciated, the complications of structure which dis- 

 tinguish them, could not be so strongly insisted upon as evidence of their typical 

 difference, as is done by Leuckart and his followers. 



Before proceeding, I would remind the reader of the little value which numerical 

 difterences undoubtedly have in this question, notwithstanding the constancy of the 

 number of parts in most of the Radiates; for though the number five is the typical 

 number among Echinoderms, there are Crinoids and Starfishes, and even Echinoids, 

 with four and six spheromeres, and others with an unusually large number; and 

 though the number four and multiples of four are the typical numbers of Acalephs, 

 we find those which have five and six spheromeres, and other numerical combi- 

 nations. We need, therefore, not hesitate to compare an Am-elia witli a quadri- 

 partite and an Echinarachnius with a quinquepartite arrangement of their parts; 

 and I trust that at least upon that ground, no exception may be taken to the 

 conclusions at which I have arrived. 



The first question to which I would call attention is, whether Aurelia consists 

 of eight or of four spheromeres. At first sight it would seem unquestionable, that 

 there are eight equivalent rays in the body of an Aurelia or Cyanea, all having 

 an eye at their peripheric termination, but four and four of which, alternating with 

 one another, differ in supporting an oral appendage and a sexual pouch. If, how- 

 ever, the peculiarities of other families are taken into consideration, it will at once 

 appear that neither the presence nor the position of the eyes, is in itself sufficient 



