PLANKTONIC STUDIES. 611 



in the tropical and subtropical seas attracted the attention of seafarers 

 by their immense numbers as well as by the irreg-iilarity of their sudden 

 ai)pearauce and disappearance. Rarer is a purely ^%so«ecfic i>lank- 

 ton chiefly composed of ForskaUa; I have observed such repeatedly at 

 Lanzarote. At that same place also occurred frequently a monotonic 

 ctenojjhora-planJcton. These delicate nettle animals also, as is well 

 known, like the Meduscv and Siphonophores, appear in such closely 

 packed crowds that there 4s scaixjely room between them for otlier 

 pelagic animals, l^ot infrequently the great accumulation of a single 

 species of ctenophore imparts to the plankton a very remarkable char- 

 acter, and this is true in all oceans, in the cold as Avell as in the warm 

 and temperate zones. More often it happens that the monotonic cnid- 

 aria-plankton is composed of several species of Meduscv, Sijjhonophores, 

 and Cteiiophores, while other classes of animals take only a very limited 

 share in its constitution. 



5. Monotonic iSagittidcc-Planlion. — The only form of monotonic plank- 

 ton wliich the branch of Helminthes furnishes is made up by the class 

 of the GJuvfognafhd, various species of the genera Sagitta and SpadcUa. 

 Although purely oceanic according to their mode of life, yet they occur 

 numerously in the neritic tow-stufl" {Anftrieb). Sometimes only a single 

 species of these genera, sometimes several species close together, 

 appear in such swarms as to make up more than half of the entire 

 jilankton. These phenomena have been observed in the colder as well 

 as in the warmer seas. In the former the plankton is composed of the 

 smaller, in the latter of the larger species. These forms occur also in 

 the deep sea, and indeed the zonary sagittida'-planMon is composed 

 of different species from the pelagic. 



6. Monotonic Ptcropoda-PlanMon. — Astonishing masses of oceanic pte- 

 ropods are very widely distributed in all i^arts of the ocean, and in part 

 are formed of characteristic genera and species in the different zones. 

 The immense schools of Clio horeaJls and Limacina arctiea, which 

 inhabit the northern seas and (as "'whale-food") furnish the chief 

 food supply for many cetaceans, sea-birds, fishes, and ceplialoi)ods, 

 have long been known. But no less immense are other swarms of 

 pteropods, composed of different genera and species, which i)oi)ulate 

 the seas of the temperate and tropical zones. These have often esca])ed 

 the notice of seafarers, because most species are nyctipelagic. Of the 

 immense quantities of these floating snails, direct evidence is furnished 

 by the accumulated calcareous shells, which in many stretches of ocean 

 (especially in the tropical zone) thickly cover the bottom at depths 

 between 500 and 1,500 fathoms. Often the greater part of this 

 "pteropod-ooze '' is formed solely of them (G, p]). 120, 922). At Messina 

 as well as at Lanzarote I found the pteropod-i)lankton often mixed with 

 considerable numbers of heteropods. Still the latter never form the 

 greater part of the volume. 



