PLANKTONIC STUDIES. 595 



Medusw. — Tlie great interest wliieli I have felt in this wonderful 

 class of animals since my first acquaintance with living medusip, in 

 1854, and which has been increased by my numerons sea voyages, led 

 me to the monographing of them (1879). I immediately gained thereby 

 a number of definite chorologicaPaud oeeological ideas, which have 

 been of permanent iufiuence in the further course of my plankton 

 studies. By it was definitely fixed the knowledge that the whole race 

 of the medusje is poIyphyletiCj and that on the one side the Graspedota 

 (or Hi/fJromednsa') have arisen independently from the Hydropolyps, just 

 as on the other side the Acraspedota (or ^Scyphoniednscv) from the iScypho- 

 polyps. In both analogous cases the transition to the pelagic, free- 

 swimming mode of life has led to the formation, from a lower, sessile, 

 very simply organized benthic animal, of a much higher planktonic meta- 

 zoon, with differentiated tissues and organs — a fact which is of great 

 significance for our general understanding of the phyiogeny of tissues. 



I have in that monograph broadly distinguished two principal forms 

 of ontogeny or individual developmental history among the medusfe, 

 meiagenesis and hypogenesis. Of these I regard metagenesis, the alter- 

 nation of generations with polyps, as the primary or paling enetic form; 

 on the other hand, hypogenesis^ the "direct development" without alter- 

 nation of generations, as the secoiulary abbreviated or cenogenetic form. 

 This distinction is of great importance in the chorology, in so far as the 

 great majority of the oceanic medusoe are liypogenetic; the neritic, on the 

 other hand, are metagenic. To the oceanic medusai in the widest sense 

 I refer the Trachylince {Trachymedtisw and Narcomedus(c) among the 

 Craspedota; to the neritic, the LeptoUnm {Anthomeduscc and Lepfome- 

 dus(v: comp. 29, p. 233). While the former have lost their relation to 

 the benthonic polyps, the latter have retained it through heredity. The 

 same seems to obtain also for the majority of the Acraspedota., namely 

 the THscomcdtisw. Among these there are only a few oceanic genera 

 with hypogenesis, e. g., Pelagia. The development of the smaller but 

 very important acraspedote orders, which I have distinguished as iStau- 

 romeduscc, Pero^nedusce, and Gnhomedusw, is, I am sorry to say, as yet 

 quite unknown. The first is to be regarded as neritic and metagenic; 

 the two latter, on the other hand, oceanic and hyx)ogeuic. That the 

 majority of the large Biscomedvsce are neritic and not oceanic is shown 

 from their limited local distribution. 



Although ten years ago the Mednsw were generally held to be purely 

 pelagic animals, it has now been found that a certain (perhaps consid- 

 erable) part of them are zonary or bathybic. Among the 18 deep-sea 

 medusw which 1 have described in part xii of the Challenger Keport 

 (1881) there are, however, some forms which occur also at the sur- 

 face, and a few which perhaps were accidentally taken in the tow net 

 while drawing it up. r>ut others are certainly true deep-sea dwellers, 

 as the Fectyllidw among the Graspedota., the PeriphylUdw and Atollidce 



