[3] MEDUSA FROM THE GULF STREAM. 929 



very great. I can at present imagine no place on the globe wliere the 

 uniformity of conditions under which medusae are placed can be the 

 same as at great depths of the ocean. I do not mean necessarily on the 

 floor of the ocean, since that may be raised or depressed, and the vari- 

 eties of conditions which come from such motions may result, but in the 

 depth of the sea, separated from the surface by a wall of water of great 

 depth, and from the ocean-bed by a similar wall of equal amount. Here, 

 if anywhere, may we look for the continuance of ancestral features un- 

 modified by environment. On this account the determination of the 

 bathymetrical limits of free medusae no less than that of those animals 

 which inhabit the bottom is a most important thing, and from it should 

 be eliminated all i^ossibility of error. I have been struck, in looking 

 over deep-sea medusse with the predominance of those which are placed 

 in the Acraspeda as compared with hydroid gonophores. This condi- 

 tion may spring from the fact that the former are larger and more easily 

 seen than the latter, or from a lack of hydroid gonophores at great 

 depths. Several observers have already noticed the predominance of 

 the Plumularidse and allied Sertularian genera at great depths, while 

 hydroids with free medusiform gonophores, as far as our knowledge 

 now goes, do not form a large share of deep-sea life. The Plumularidse 

 have no free medusiform gonophores, as far as known. 



Of the medussB placed among the Acrasi^eda which I haye studied the 

 majority are allied to the simpler genera represented by the family of 

 EphyridfB. These are regarded as closely approaching the ancestral 

 form, Ephyra, from which a great group of medusae has sprung. If it 

 should be found, on a larger acquaintance with these animals, that they 

 conform to the law which they seem to indicate, we may have new fac's 

 of greatest importance in the study of the phylogeuy of the medusie. It 

 is self-evident, I think, that if medusai are confined to certain depths 

 and cannot penetrate below them in younger or adult stages, a develop- 

 ment without a fixed Scyphostoma stage must be the only means of de- 

 velopment from the egg in these genera. We may find, as in Gimina, 

 commensalism or parasitism, or even a free nurse akin to that of the 

 '•mother bud" of Guniria, as recorded by Metschnikoff and others; but 

 the possibility of the depth below which the animal cannot sink being, 

 the same as the depth of the bottom would be doubtful. What arc the 

 possibilities of a medusa separated by a zone of 1,000 fathoms of water, 

 through which it is supposed that it cannot penetrate, being able to 

 make its way to places of that depth at which it lives for its Scypho- 

 stoma to become attached ? If we say that the eggs may be dropped 

 and sink to the floor of the ocean, no matter how deep, can we still hold 

 to the proposition that the medusa is limited to any depth? A priori 

 then, if medusse are confined to a certain depth, we must suppose that 

 they can have an attached Scyphostoma only when brought, as jilauulse 

 or otherwise, into those regions of the ocean whose floor comes within 

 their bathymetrical zones. 

 H. Mis. 68 59 



