PLANKTONIC STUDIES. 595 
Meduse.—The great interest which I have felt in this wonderful 
class of animals since my first acquaintance with living meduse, in 
1854, and which has been increased by my numerous sea voyages, led 
me to the monographing of them (1879). ITimmediately gained thereby 
-a number of definite chorological and cecological ideas, which have 
been of permanent influence in the further course of my plankton 
studies. By it was definitely fixed the knowledge that the whole race 
of the meduse is polyphyletic, and that on the one side the Craspedota 
(or Hydromeduse) have arisen independently from the Hydropolyps, just 
as on the other side the Acraspedota (or Scyphomeduse) from the Scypho- 
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 penthic animal, of amuch higher planktonic meta- 
zoon, With differentiated tissues and organs—a fact which is of great 
significance for our general understanding of the phylogeny of tissues. 
I have in that monograph broadly distinguished two principal forms 
of ontogeny or individual developmental history among the medusze, 
metagenesis and hypogencsis. Of these I regard metagenesis, the alter- 
nation of generations with polyps, as the primary or palingenetic form; 
on the other hand, hypogenesis, the ‘‘ direct development” without alter- 
nation of generations, as the secondary abbreviated or cenogenetic form. 
This distinction is of great importance in the chorology, in so far as the 
great najority of the oceanic meduse are hypogenetic; the neritic, on the 
other hand, are metagenic. To the oceanic meduse in the widest sense 
I refer the Trachyline (Trachymeduse and Narcomeduse) among the 
Craspedota; to the neritic, the Leptolinw (Anthomeduse and Leptome- 
duse: 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 Discomeduse. 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 Stau- 
romeduse, Peromeduse, and Cubomeduse, is, 1 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 Discomeduse@ 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 (perhaps consid- 
erable) part of them are zonary or bathybic. Among the 18 deep-sea 
meduse which I have described in part x1 of the Challenger Report 
(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 Pectylide among the Craspedota, the Periphyllide and Atolliide 
