PLANKTONIC STUDIES. 567 
‘“Miiller compares the different methods of obtaining them, and chooses, 
above all, “fishing with a fine net at the surface of the sea.” He 
Says: 
I have used this method for many years with the best results; for the advanced 
stages of the swimming larve aud for the time of maturity and metamorphosis it 
is quite indispensable, and in no way to be replaced. 
The students who, in 1845-46, as well as in the following years, 
accompanied Johannes Miiller to Helgoland and Trieste (Max Miiller, 
Busch, Wilms, Wagener, and others) were introduced into this method 
of “pelagic fishery” and into the investigation of “pelagic tow-stulf” 
(pelagische Auftrieb) obtained thereby. It was soon employed at sea 
with excellent results by other zoédlogists—by T. H. Huxley, by Krohn, 
Leuckart, Carl Vogt, and others, and especially by the three Wiirts- 
burg naturalists, A. Kolliker, Heinrich Miiller, and C. Gegenbaur, 
who in 1852 examined with such brilliant success the treasures of the 
Straits of Messina. At this time, in the beginning of the second half 
of our century, the astonishing wealth of interesting and instructive 
forms of life which the surface of the sea offers to the naturalist first 
became known, and that long series of important discoveries began 
which in the last forty years have filled so many volumes of our rapidly 
increasing zoélogical literature. A new and inexhaustibly rich field 
was thus opened to zodtomical and microscopical investigation, and 
anatomy and physiology, organology and histology, ontogeny and 
systematic zodlogy have been advanced to a surprising degree. The 
investigation of the lower animals has since then been recognized as 
a wide field of work, whose exploration is of great significance for all 
branches of science and to which we owe numberless special and the 
most important general conclusions. 
The general belief of zodlogists regarding the extent of this rich 
pelagic animal world arose as the result of the discovery that a special 
“pelagic fauna” exists, composed of many characteristic forms, funda- 
mentally different from the littoral fauna. This pelagic fauna is made 
up of animals (some floating passively, others actively swimming) which 
remain at the surface of the sea and never leave it, or only for a short 
time descend to aslightdepth. Among such true “pelagic animals” 
are the radiolaria, peridinia, noctiluca, medusze, siphonophores, cten- 
ophores, sagitta, pteropods, heteropods, a greater part of the crustacea, 
the larvie of echinoderms, of many worms, ete. 
Important changes were first made in the prevailing idea of the 
“pelagic fauna” by the remarkable discoveries of the epoch-making 
Challenger expedition (1873-1876). The two leaders of this, Sir 
Wyville Thompson and Dr. John Murray, did not limit themselves to 
their chief object, the general physical and biological investigation 
of the deep sea, but studied with equal care and perseverance the 
conditions of organic life at the surface of the ocean and in zones of 
