‘ PLANKTONIC STUDIES. Gig 
in the tropical and subtropical seas attracted the attention of seafarers 
by their immense numbers as well as by the irregularity of their sudden 
appearance and disappearance. Rarer is a purely physonectic plank- 
ton chiefly composed of Forskalia; I have observed such repeatedly at 
Lanzarote. At that same place also occurred frequently a monotonic 
ctenophora-plankton. These delicate nettle animals also, as is well 
known, like the Medusew and Siphonophores, appear in such closely 
packed crowds that there is scarcely room between them for other 
pelagic animals. Not 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 well as in the warm 
and temperate zones. More often it happens that the monotonic enid- 
aria-plankton is composed of several species of Meduswe, Siphonophores, 
and Otenophores, while other classes of animals take only a very limited 
share in its constitution. 
5. Monotonic Sagittide-Plankton.—The only form of monotonic plank- 
ton which the branch of Helminthes furnishes is made up by the class 
of the Chetognatha, 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 (Auftrieb). 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 
plankton. 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 sagittide-plankton is composed 
of different species from the pelagic. 
6. Monotonic Pteropoda-Plankton.— Astonishing 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 borealis and Limacina arctica, which 
inhabit the northern seas and (as ‘*‘whale-food”) furnish the chief 
food supply for many cetaceans, sea-birds, fishes, 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 temperate and tropical zones. These have often escaped 
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 (6, pp. 126,922). 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. 
