PLANKTONIC STUDIES. 615 
gulf was filled by crowds of a new, hitherto undescribed, large medusa, 
Drymonema cordelia. Thousands of these Cyaneidw lay cast upon the 
beach at Cordelio.* 
Monthly oscillations.—The time of year is of just as great importance 
for the appearance of very many pelagic animals as for the flowering - 
and fruit formation of land plants. Many of the larger planktonic ani- 
mals, Meduse, Siphonophores, Ctenophores, Heteropods, Pyrosoma, ete., 
appear only in one month or during a few months of the year. They 
form Hensen’s “periodic plankton.” In the Mediterranean many 
pelagic animals are numerous in the winter, while in the summer they 
are entirely wanting. This “periodical appearance of pelagic animals” 
has long been known and often mentioned; but not so the important 
fact that these ethoral periods themselves show considerable variations. 
For this the tables of Schmidtiein (19) and the notes of Graeffe (20) 
give important points of support. Especially the Disconecte and other 
Siphonophorest behave very irreguiarly. The cause of the monthly 
variation lies on the one side in the conditions of reproduction and 
development; on the other in the varying temperature of the season, 
as Chun has lately shown (15, 16). 
Daily oscillations—Every naturalist who has observed and fished 
pelagic animals and plants in the sea for a long time, knows how unlike 
their appearance is on different days in the same period of the year or 
in the same month, when one may daily hope to find them. Asa rule, 
the weather, and particularly the wind, conditions the remarkable 
difference of appearance. In long-continuing calms the surface of the 
sea becomes covered with swarms of various pelagic creatures. In 
long bands, smooth as oil, the most wonderful zodcurrents appear. 
But as soon as a fresh wind stirs up lively waves, the majority sink 
into the quiet depths, and if a more violent storm churns up the deeper 
layers, all life vanishes from the surface for days. Many animals of 
the plankton (especially oceanic) are very susceptible to the influence 
of fresh water, and therefore disappear during violent rains. Warm 
sunshine entices the one to the surface, while it drives the other into 
thedepths. ‘This influence of the weather upon the quality and quantity 
of the planktonic composition is so well known that it is not necessary 
to give examples. Hensen (9) has even gone over his work many times, 
without thinking how the above endangers his “exact methods” and 
made their results illusionary. 
* Drymonema cordclia, whose milk-white umbrella reaches half a meter in diameter, 
TI will describe hereafter. It differs in the formation of the gonads and oval tenta- 
cles, as in several other points, from the Adriatic species, which I have described as 
Drymonema victoria (=dalmatinum) (11, 29). 
tOFf the Disconecte (Porpila and Velella) Chun during a 7 months’ residence at 
the Canary Islands (1887-88) could find not a single specimen. According to him 
they should appear first in midsummer (July to September). On the other hand 
I saw at Lanzarote an isolated swarm of these Disconectw in midwinter, in Feb- 
ruary, 1867, 
