Chap. II. 



HOMOLOGICAL RELATIONS. 



71 



Fig. 6. 



ence consisting in the greater complication of the ambulacral system of the Encope, 

 and in the presence of five ambuhicra, whereas AureUa has only four. But when 

 it is remembered how simple the ambulacra of Synapta are, and how great a diver- 

 sity exists in the relative development of the ambulacral and interambulacral zones, 

 throughout the type of the Radiates, such differences cannot be considered as 

 impairing the homology of these parts. 



A further comparison with Melitta quinquefora, Fi(). 6, will only confirm these 

 conclusions, and, I trust, also go far to show how little foundation there is for a 



typical separation of the Coelenterata 

 and Echinodermata. In this figure the 

 ambulacral and interambulacral systems 

 are seen from the inside of the lower 

 floor of the spherosome, I, II, III, IV, 

 and V representing the ambulacral sys- 

 tem, and A, B, C, D, and E the inter- 

 ambulacral system of radiating tubes, 

 and a and b, and i and 2, for their 

 respective ambulacral and interambula- 

 cral zones, the branches by which they 

 anastomose with one another. There 

 is, in this genus, as well as in the genera 

 Dendraster and Echinarachnius, Fig. 3, 

 an additional point of correspondence 

 with Aurelia, not observed in Encope : 

 in the interambulacral zones may be seen 

 two simple tubes (i and 2) bordering 

 upon the wider pouches, facing A, B, C, 

 D, and E, into which the sexual organs 

 extend. The innumerable lacunar in the peripheric portion of the spherosome are 

 only dilatations of the radiating tubes, and might at first sight appear to have little 

 resemblance to the chymiferous tubes of the Acalephs ; but if, instead of comparing 

 the mode of ramification and the combinations of these lacuna? with the ramifications 

 of the chymiferous system of Aurelia, we turn to Polyclonia, as represented PI. XIII. 

 Fig. 2, or to Rhizostoma, as represented by Milne-Edwards,^ the resemblance is most 

 striking, and I am satisfied that there is no exaggeration in the statement I made 

 before, that Echinoderms are Acalephs with a somewhat more complicated organ- 



MeLITTA QUISqUEFUKA. 



I, II, III, IV, V, ambulacral system, — A. B. C, D. E, interambulacral syBtem. 

 — a, 6 and 1, 2, the respective halves of these systems. 



* See Recherches Anatomiques et Zoologiques 

 faites pendant un Voyage sur les cotes de la Sicile, 



Part I. PI. I., or Cuvier's Regne animal, illus- 

 trated edition, Zoophytes, PI. 50. 



