PLANKTONIC STUDIES. 615 



gulf was tilled by crowds of a new, hitherto undescribed, large medusa, 

 Dri/inonema corddia. Tliousands of those Cyaneidic lay cast upon the 

 beach at Cordelio.* 



Monthly oscillatluns. — The time of year is of just as great importauce 

 for the appearance of very many pelagic animals as for the flowering 

 and fruit forinatioii of land plants. Many of the larger planktonic ani- 

 mals, 2Icdus(V, Siplionophores^ Gtenophorefi^ Jlcferopods, Pyro.soma, etc., 

 appear only in one month or during. a few months of the year. They 

 fiU'iii Hensen's '"^periodic plankton." In the Mediterranean many 

 pelagic animals are numerous in tlie winter, Avhile 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 Schmidtlein (19) and the notes of Graeffe (20) 

 give important points of supi^ort. Especially the Disconectw and other 

 SiphoH02)hores\ behave very irregularly. The cause of the monthly 

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

 development; on the other in the varying temx^erature 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. As a rule, 

 the weather, and particularly the tvind, conditions the remarkable 

 difference of appearance. In long-continuing calms the surface of the 

 sea becomes covered with swarms of vari(ms pelagic creatures. In 

 long bands, smooth as oil, the most wonderful zoik-urrents 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 tlie one to the surface, wliile it drives the other into 

 the depths. This influence of the weather upon the quality and quantity 

 of the plaid^tonic composition is so well known that it is not necessary 

 to give examples. Hensen (0) has even gone over his work many times, 

 without thinking how the above eiulangers his "exact methods" and 

 made their results illusionary. 



* Drymonema cordella, whoso milk-white uiiihrella loiichns half a meter in diameter, 

 I will describe hereafter. It diffc^rs in the formation of the gonads and oval tenta- 

 cles, !is in .several other points, from the Adriatic^ .si)eci('s. wliich I have described as 

 Drymoncma victoria (=dahna1inHm) (ii, 29). 



tOf the I)ixci>iiect.(r, (Porpila and J^elella) Chnn dnrin<j a 7 months' residence at 

 the Canary Islands (1887-SH) conld find not a single specimen. According to him 

 they should ui)pear lirst in midsummer (July to September). On tiie other hand 

 I saw at Lanzarotc an isolated swarm of these Disconevla; in midwinter, in Feb- 

 ruary, 1867. 



