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PROCEEDINGS OF THE 



[Dec. 1857. 



mentioned that the anal pores of the Discophora belong to this 

 same system of tubes, which may be thus homologised with the 

 two anal pores, one on each side of the sense-capsule in Ctenophora. 



How does it then happen that these tubes which, among Disco- 

 phora, belong, as it were, to the same horizontal plane, are among 

 Ctenophora so variously directed, one ascending perpendicularly 

 towards the sense-capsule, two descending towards the mouth, 

 (where, like the rest, they bifurcate,) and four or six directing 

 themselves horizontally, at least, at first ? How does it occur that, 

 of all of them, only a single one ascends towards the homologue 

 of the margin of the disk in the lower orders? This would be a 

 great difficulty in this explanation, were it not that we have an 

 evident tendency among Discophora to the distribution of the 

 marginal capsules, and the tentacula upon distinct though con- 

 centric circles. And, also, it should be remembered that the anus, 

 which among ^steridse is dorsal, appears among Echini proper 

 frequently in the inter-ambulacra, belonging to the lower surface, 

 and sometimes even is extremely near the mouth. This shows 

 that the arrangement of internal systems of organs is not wholly 

 dependent upon the arrangement of the external surface. And 

 because the upper surface of the Medusa, among Ctenophora, is 

 reduced almost to nullity, and confined to the upper pole of the 

 body, is no reason why some of the radiate tubes should not pre- 

 serve their old horizontal position. 



In the Order of Ctenophora there appear to be two sub-orders ; 

 distinguished by the presence or absence of tentacula.* Thus 

 tentacula which are so conspicuous in Cydippidae, and are found 

 also in a large number of allied genera as Pleurobrachia, Mnemia, 

 Bolina, Eucharis and Cestum, appear to be entirely absent in Be- 

 roe and Medea, if the latter be not the young of Beroe. This 

 distinction appears, as I have related above, at a very early age ; 

 for I have not yet seen any stage of Bolina so young as not to pos- 

 sess tentacula, yet embryos of Beroe, not much older than the 

 younger stages of my Bolina embryos, possessed no vestige of 

 them. It is probable, therefore, that this is a fundamental embry- 

 ological distinction between the two groups. It is worthy of note, 

 also, that it is the Beroidae which possess the appendages of the 

 area around the sense-capsule, and that these, so far as 1 am 



*Had Eschscholtz been aware of the fact that Mnernidae possess tentacula, he 

 would probably have united them with his Cydippidae; for it is only on this 

 ground that he separates them. 



