PLANKTONIC STUDIES. 595 



Me(Jusa\ — Tlie great interest wliicb I have felt in this wonderful 

 class of animals sin<'e my first acquaintance with living medusjc, in 

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

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

 a number of definite chorological and (Ecological ideas, which have 

 been of permanent influence in the further course of my plankton 

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

 of the medusie is polijphi/Jctic, and that on the one side the GraHpcdoia 

 (or Ili/dromedusa') have arisen independently from the Hyclropolyps, just 

 as (^n the other side the Aeraspedota (or Scypliomcdnsw) from the kSeyplio- 

 poJyps. 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 plauktonic meta- 

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

 significance for our general understanding of tlie jjliylogeny of tissues. 



I have in that monograph broadly distinguished two principal forms 

 of ontogeny or individual developmental history among the medusae, 

 metagenesis and hypogenests. Of these I regard metagenesis, the alter- 

 nation of generations with polyps, as the primary o\ paUngenetlc form; 

 on the other hand, hypogenesis, the "direct develoiDment " without alter- 

 nation of generations, as the secondary abbreviated or cenogcneUc form. 

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

 great majority of the oceanic medusjc are hypogenetic; the nentic, on the 

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

 I refer the TracJiylinw {Trachymedusw jyid Narcomedusa') among the 

 (Jraspedota; to the neritic, the Leptolincc {Anthomedusw and Leptome- 

 dusev: 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 Aeraspedota, namely 

 the JHseomedusa\ 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 ISfau- 

 romedusa\ Peromeditsa^, and Cuhomedusw, 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 hypogenic. That the 

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

 from their limited local distribution. 



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

 pelagic animals, it has now been found that a certain (j^erhaps consid- 

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

 medusce which I have described in part xii of the Challenger Eeport 

 (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. But others are certainly true deep-sea dwellers, 

 as the Pectyllidev among the Oraspedota, the PeriphylUdw and AioUidw 



