﻿Dec, 1857.] 



ELLIOTT SOCIETY. 



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homology between Echini and Asteridae has been very ably shown 

 by Prof. Agassiz. 



To understand this homology we have only to imagine a Dis- 

 cophore with a very short digestive trunk or none at all, that is 

 with the digestive cavity in the disk. If now we suppose the 

 outer or upper surface of the disk to be very much contracted, in 

 fact reduced to a mere oblong area on the top of the body of the 

 animal, while at the same time the inner or under surface of the 

 disk or bell is very much developed, and instead of remaining 

 concave becomes convex, and covers nearly the whole external 

 surface of the animal ; it is obvious that by such a change the 

 margin of the disk would be carried upwards, and being con- 

 tracted at the same time, would at last appear as a small circle, 

 surrounding the superior pole of the body; with the margin, 

 would be carried upwards all the marginal appendages, such as 

 the sense-capsules and the tentacula or their representatives. 

 Now such appears to be actually the case in Beroe. For its 

 sense-capsule at the superior pole is so little different in appear- 

 ance from the marginal capsules in the Discophora that the re- 

 semblance must have struck every one conversant with the three 

 orders, and it is by no means impossible to suppose that the short 

 more or less branched appendages of the oblong area in that 

 genus, are the homologues of structures in the lower Medusae. 

 For in several genera among Discophora, besides the tentacula 

 proper, the margin of the disk is fringed all round with short ap- 

 pendages. In Cyanea ferruginea, Eschscholtz has represented 

 them (pi. 5, fig. la. Acaleph,) as more or less arborescent, which 

 seems to be their tendency morphologically. But Huxley has 

 described particularly the peculiar heart-shaped area, situate just 

 above each of the sense-capsules in Rhizostoma, (Phil. Trans. 1851 

 and Annales des. Sci. Nat. 3me. Ser. vol. xv. p. 337.) Of this 

 structure he remarks that " La surface proemine sous forme de 

 plis arborescents, et tres abondamment cilies.' The same ex- 

 pressions might be made use of to describe the elongate area in 

 Beroe, with the exception that the arborescent appendages there 

 are confined to the margin of the area. But we should also expect 

 that the direction of the tubes which radiate from the centre 

 towards the periphery in the lower medusae, should follow the 

 tentacular margin upwards, and hence their direction changed 

 from a more or less horizontal to a vertical one. And accordingly 

 we find in the embryo described by Kolliker, that the first tube 

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