PLANKTONIC STUDIES. 615 



g-ulf was tilled by crowds of a new, hitherto uudescribed, large medusa, 

 Drymonema eordelia. Thousaiid.s of these Cijaneidw lay cast upon the 

 beach at Cordelio.* 



MoHthUj oscillations. — The time of year is of Just as great imi^ortauce 

 for the appearance of very many i>elagic animals as for tlie liowering 

 and fruit formation of laud j)lauts. Many of the larger planktonic ani- 

 mals, Medusw, Siphonophores, Gtenophores, Heteropods, Fi/rosoma, etc., 

 appear only in one month or during a few montlis 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 ai)pearance of pelagic animals" 

 has long been known and often mentioned; but not so tl^e important 

 fact that these ethoral periods themselves show considerable variations. 

 For tliis the tables of Schmidtlein (19) and the notes of Graeffe (20) 

 give important points of support. Especially the DisconecUv and other 

 Siphonophores\ behave very irregularly. The cause of the monthly 

 variation lies on the one side in the conditions of reproduction and 

 develoi>ment; on the other in the varying temperatTU-e of the season, 

 as Chun has lately Shown (15, 16). 



Daily oscillations. — Every naturalist wh.) has observed and fished 

 pelagic aninuils and plants in the sea fov a long time, knows how uidike 

 their appearance is on different days in the same period of the year or 

 in the same mouth, when one may daily hope to find them. As a rule, 

 the weather, and particularly the wind, conditions tlie remarkable 

 <liflerence 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 zoorurrents appear. 

 Bat as soon as a fresh wind stirs up lively waves, the majority sink 

 into the quiet depths, and if a more violent storm churns u^) the deeper 

 layers, all life vanishes from the surface for days. Many aninmls of 

 the plankton (especially oceanic) are very suscejitible to the infiuence 

 of fresh Avater, and therefore disappear during violent rains. Warm 

 sunshine entices the one to the surface, while it drives the other into 

 tiie depths. 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 

 nmde their results illusionary, 



* Driihionema eordeVm, whose milk-white iiml)rella. reache.s halfii meter in diameter, 

 I will describe liereafter. It differs! in the formation of tlie gonads and oval tenta- 

 cles, as in several other points, Irorn the Adriatic species, which I have described as 

 Drtjmonema victoria {^= (lalmaiinum) (ii, 29). 



f Of the Disconectce (Poiyita and VcJella) Chun during a 7 months' residence at 

 tlie Canary Islands (1887-88) conld 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 Diseonectw in midwinter, in Feb- 

 ruary, 1867. 



