﻿266 



PROCEEDINGS OF THE 



[Dec, 1857. 



developed in the young Beroid is the vertical one passing upward 

 from the central chamber of the circulatory system to the sense- 

 capsule at the superior pole. 



Here we are met by a seeming difficulty in the great reduction 

 in the number of parts which are numerously represented in the 

 lower groups. There is but a single sense-capsule in the Ctenophora 

 to correspond to the eight or more of the Discophores. There is, 

 likewise, but a single vertical tube to correspond to the typical 

 four in the lower groups. While among Beroidae, the appen- 

 dages which we have spoken of as representatives of tentacula, 

 disappear altoge-her. But it should be remembered that such 

 reduction of the numbers of similar parts is common in passing 

 from a lower group to a higher one in the animal kingdom. 

 Solaster has numerous ambulacra, and Crinoids have divaricate 

 ones, yet they are all homologous with the five ambulacra of 

 Spatangus, or Echinus. It must be acknowledged, however, that 

 if this homology be the correct one, the reduction of numbers in 

 the case of the sense-capsule and the vertical tube, is remarkably 

 great. 



In regard to the remaining parts, with the exception of the 

 digestive cavity, this view of the homologies of medusae would 

 lead us to suspect that they have no immediate homologues among 

 the medusae of the lower orders. Such are the two main lateral 

 divaricate horizontal trunks of the circulatory system, the gastric 

 tubes, the ambulacral tubes and the ambulacra themselves, besides 

 the tentacula of Cydippidae. With the exception of a portion of the 

 ambulacra and the tentacula of Cydippidae, whose time of de- 

 velopment is uncertain, it is known that these parts are developed 

 after those which I have attempted to homologise with the parts 

 of the lower medusae. It is possible, therefore, that they are pe- 

 culiar altogether to the Ctenophora and belong to a different order of 

 things from that which is exhibited in the Discophores and Hy- 

 droids ; that they are special structures, whose appearance is 

 connected with the vast development and bulging out of the lower 

 portion of the disk. The tubes, therefore, of this part of the body 

 and the tentacula of Cydippidae, would, in this view, be only 

 serially homologous with the radiate tubes and tentacula of 

 Discophora and Hydroids, while the ambulacra of ciliary blades* 



*K6llikers notion that they are homologous with the cilia of the planule, appears 

 to me inadmissible, since, according to my view, they belong to the homologue 

 of the inner surface of the bell. 



