PLANKTONIC STUDIES. 611 



ill the tropical and subtropical seas attracted the attention of seafarers 

 I)y their immense numbers as well as by the irregularity of their sudden 

 appearance and disappearance. Earer is a purely 'physonectic plank- 

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

 Lanzarote. At that same place also occurred frequently a monotonic 

 cteno2>hora-pl(inkton. These delicate nettle animals also, as is well 

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

 packed crowds that there is scarcely room between tliem for other 

 pelagic animals. Not infrequently the great accumulation of a single 

 species of ctenophore imparts to the planktoii a very reuiarkable char- 

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

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

 aria-plankton is composed of several species of Medusw, Siphonophores, 

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

 share in its constitution. 



5. Monotonic Sagittkkv-Phmlion. — Tlie only form of monotonic plank- 

 ton which the branch of Helminilies furnishes is made up by the class 

 of the Cha'tognatha, various species of the genera Sagitta and Spadella. 

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

 numerously in the neritic tow-stuff {Auftrieh). Sometimes only a single 

 species of these genera, sometimes several species close together, 

 appear in such swarms as to make n[) more tlian half of the entire 

 plankton. These phenomena have been ol)served in tlie colder as well 

 as in the warmer seas. In tlie former tlie 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 HagHtUhv-planMon is composed 

 of different species from the pelagic, 



f). Monotonic Pteropoda-Planldon. — /vstonishing masses of oceanic pte- 

 ropods are very widely distributed in all parts of the ocean, and in part 

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

 The immense schools of Clio horcfdis and Limacina arctica, which 

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

 food supply for many cetaceans, sea-birds, tishes, and cephalopods, 

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

 pteropods, composed of different genera and species, which populate 

 the seas of the temi)erate and tropical zones. These have often escaped 

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

 immense quantities of these floating snails, direct evidence is furnished 

 by theaccunnilated 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, pp. 120, 922). At Messina 

 as well as at Lanzarote I found the pteropod-plankton often mixed with 

 considerable numbers of heteropods. Still the latter never form the 

 greater part of the volume. 



