PLANKTONIC STUDIES. 587 



•4. Diatomeiv. — The iiicouceiv^iible quautitie.s in whicli tlie diatoms 

 populate the whole ocean and the extraordinary importance which they 

 possess as one of the most imjiortant constituents of the "fundamental 

 food supply" {Uriiahrung) in the cycle of matter in the sea has been 

 considered so many times that it is sufihcient here to point to the com- 

 paratively recent accounts of Murray (.">, p. 533; G, p. 737, etc.), Fuchs 

 (12, p. 49), Oastracane (6, p. 930), and Hensen (9, p. 80). Earlier the 

 chief attention was paid to the benthonic diatoms which everywhere 

 cover the seacoast and the shallow depths of the sea bottom in aston- 

 ishing quantities; in part fixed on stalks, in part slowly moving among 

 the forests of seaweed and the fixed animal banks {festsitzenden Thier- 

 hanlcen) of the coast. The importance of the planktonic diatoms was 

 recognized much later, those abounding in the open ocean as well as in 

 the coast waters furnishing one of the most important sources of food 

 for the i)elagic animals. The oceanic diatoms, which often cover the sur- 

 face of the open sea as a thick layer of slime, form another flora, very 

 insufficiently studied and characterized by many forms of colossal size 

 (several millimeters in diameter), peculiarly regular in form, and with 

 extremely thin-walled siliceous shells (species of JEthmodiscus, Goscino- 

 discus, liJiizosoleiiia, etc., discovered in such numbers by the Challenger). 

 The ncritic diatoms, on the other hand, which, swimming free in no 

 small numbers, populate the coast waters, are less in diameter and with 

 thicker walls, and stand on the whole between the oceanic and littoral 

 forms. The absolute and relative quantity of the planktonic diatoms 

 seems to increase gradually from the equator towards both poles. 



In the tropical zone the pelagic diatoms are much less developed 

 than in the temi)erate zone, and here again much less than in the polar 

 zone. Wide stretches of the Arctic Ocean are often changed by incon- 

 ceivable masses of diatoms into a thick dark slime, the "black water," 

 which forms the feeding-ground of Avhales. The pteropods and crus- 

 taceans, upon which these cetaceans live, feed upon this diatom slime, 

 the "black water" of the Arctic voyager. Not less wonderful are the 

 vast masses of diatoms which fill the Antarctic Ocean south of the 

 fiftieth degree of latitude, and whose siliceous shells, sinking to the 

 bottom after the death of the organism, form the diatom ooze (Challenger, 

 stations -52-157). The tow nets here were quickly filled with such 

 masses of diatoms (for the most part composed of Cha'toceros) that these 

 when dried in the oven formed a thick matted felt ((5, }). 920). 



5. Xaiithcllea\ — A highly important share in the cycle of matter in the 

 sen belongs to the remarkable xanthellece or "yellow cells," which live 

 in si/mbiosis in the bodies of many marine animals, in the plankton as 

 tvell as in the benthos. I first proved that these "yellow cells," which 

 were observed by Huxley (1851) and by Johannes Miiller (1858) in the 

 calymma of radiolarians, Avere "undoubted cells," and also described 

 their structure and increase by divisi(m (3, p. 84), and later (1870) 

 showed that they constantly contained amyluni (4, § 90). But Cien- 



