PLANKTONIC STUDIES. ^ 567 



Miiller compares the different methods of obtaining- them, and chooses, 

 above all, ''fishing- with a fine net at the snrface of the sea." He 

 says : 



I liave used this method for many years witli the best results; for the advanced 

 stages of the swimming hirva^. and for the time of maturity and metamorphosis it 

 is quite indis})ensable, 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-stuff" 

 {pelddische Aii/trieh) obtained thereby. It was soon employed at sea 

 with excellent results by other zoologists — by T. H. Huxley, by Krolm, 

 Leuckart, Carl Vogt, and others, and especially by the three Wiirts- 

 burg naturalists, A. Kolliker, Heinrich Miiller, and (3. 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 zoological literature. A new and inexhaustibly rich field 

 was thus opened to zootomical and microscopical investigation, and 

 anatomy atid physiology, organology and histology, ontogeny and 

 systematic zoology 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 zoologists regarding the extent of this rich 

 pelagic animal world arose as the result of the discovery that a s])ecial 

 "pelagic fauna" exists, composed of many characteristic forms, funda- 

 mentally different from the littoral fauna. This pelagic fauna is made 

 uj) 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 a slight dei)th. Among such true "pelagic animals" 

 are the radiolaria, peridinia, noctiluca, medusiie, siphonophores, cten- 

 ophores, sagitta, pteropods, heteropods, a greater.part of the Crustacea, 

 the larvai of echinoderms, of many worms, etc. 



Important changes Avere first made in the prevailing idea of the 

 "pelagic fauna" by the remarkable discoveries of the epoch-making 

 Challenger expedition (1873-1870). 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 



