Bathymetric distribution of the species .... 210 2. Penetration of species into the Bathymetric zonation of the deep sea ...... 210 Mediterranean deep sea ............. 235 Bathymetric distribution and taxonomic 3. Comparison with other groups ....... 235 relationship .......................... 214 Sediments and nutrient conditions ........ 235 Number of species and individuals in Hydrostatic pressure ..................... 237 relation to depth ...................... 217 Currents ............................... 238 Topography ............................ 238 E. Geographic distribution .................. 219 Conclusion ............................. 239 The exploration of the different geographic regions ............................... 219 G. The hadal fauna ........................ 239 The bathyal fauna ...................... 220 The hadal environment .................. 239 The abyssal fauna ....................... 221 General features of the hadal fauna ....... 240 1. World-wide distributions ............ 223 2. The Antarctic Ocean ................ 224 H. Evolutionary aspects .................... 243 3. The North Atlantic ................. 225 Evolution within the deep sea versus 4. The South Atlantic ................. 227 immigration from shallower depths .... 243 5. The Indian Ocean .................. 22; Geographic variation .................... 244 6. The Indonesian seas ................ 228 1. Clines ............................. 245 7. The Pacific Ocean .................. 231 2. Local variation ..................... 245 8. The Arctic Basin ................... 232 3. Polytypic species ................... 245 9. The Mediterranean ................. 232 4. Superspecies ....................... 246 The ranges of geographic distribution The alleged antiquity of the deep-sea compared with those of other groups .... 233 fauna ................................ 246 F. Distribution in relation to environmental IV. Summary .................................. 247 factors ............................... 233 Temperature ........................... 233 V. References ................................. 254 1. Faunal similarities between the polar sublittoral regions and the deep sea . 233 VI. Index to names of genera and species ........ 259 I. INTRODUCTION Among the five orders of the class Holothurioi- gation also includes the Elasipoda taken during dea, the Elasipoda are unique in being confined three collecting voyages of the late Dr. Th. Mor- to the deep sea, i. e. to depths exceeding 200400 tensen during the years 1913-1930. meters. The working up of this global material made The four remaining orders (Aspidochirota, necessary a comparison with the described ma- Molpadonia, Dendrochirota, and Apoda) all in- terials from earlier deep-sea expeditions. With clude sublittoral as well as deep-sea groups. These this purpose visits were paid to the museums of orders will be dealt with in subsequent volumes London, Amsterdam, Paris, Monaco, Washing- of the Galathea Report. ton, and Cambridge (Mass.). The Holothurioidea comprise about 1100 de- The present work discusses all the 106 known scribed species (Madsen 1954). Approximately 380 species of benthic Elasipoda, 75 of which have species belong to the deep sea. been examined. Previous to the present revision, the Elasipoda comprised 171 species. Of these, 165 belonged to Among the many people who have contributed the benthic families Deimatidae, Laetmogonidae, with aid and advice to the present work I owe a Psychropotidae, and Elpidiidae, while the remain- particular debt to the late Dr. Anton Fr. Bruun, ing six belonged to the pelagic family Pelago- the inspiring scientific leader of the Galathea thuriidae. The present investigation deals with Expedition, who gave me the opportunity to the four benthic families only. Eleven species are participate in the expedition and afterwards pla- erected, while 18 previously described species are ced at my disposal the rich material of holothuri- left out, being insufficiently described. The total ans. number of recognized benthic species is reduced I am indebted to all my deep-sea colleagues in from 165 to 106. the Zoological Museum, of whom I may mention In addition to the material brought home by Dr. F. J. Madsen, curator of echinoderms, and the Galathea Expedition (1950-l952), the investi- Dr. T. Wolff, editor of the Galathea Report. A number of echinoderm specialists have as well as that of most other deep-sea animal shown me a great helpfulness during my visits groups. The holothurians from this famous ex- to foreign museums: Dr. 6. Cherbonnier (Paris), pedition were treated by Theel (1882, 1886 a) Dr. Ailsa M. Clark (London), Professor M. Engel who erected the order Elasipoda, comprising 52 (Amsterdam), Professor N. B. Fell and Dr. Elisa- species. Only one of the species, Elpidia glacialis, beth Deichmann (Harvard), and Dr. D. L. Paw- was known before, described by the same author son (Washington). Dr. Johanne Kjennerud (Ber- in an earlier paper (ThCel 1876). gen) kindly sent specimens for re-examination. While the Challenger brought home material The photographs in Pls. I-XII were made by from the three main oceans, the subsequent deep- Mr. G. Brovad. sea expeditions explored restricted regions. The histological sections of the gonads shown The Norwegian North-Atlantic Expedition in Pls. XI11 and XIV were kindly prepared and 1876-1878, with the research vessel Voringen, photographed by Dr. J. Liitzen and Mr. Kjeld explored the Norwegian Sea and the adjacent Hansen, Institute of Comparative Anatomy, Uni- regions of the North Atlantic (Danielssen & versity of Copenhagen. Koren 1882). The genera Irpa and Kolga were Most of the drawings, apart from the simple erected on this material. line drawings, were made by the late Poul H. The Ingolf 1895-1896 explored the northern- Winther (PHW), while a few are due to Mrs. most part of the North Atlantic and the south- Lise Jersing (LJ) and Mrs. Julie Tesch (JT). western part of the Norwegian Sea (Neding 1935, The diagrams in Figs. 99-103 were made by Mr. 1942). The Godthaab 1928 explored the West E. Leenders. Greenland seas (Mortensen 1932). The English text was revised by Mrs. Manon The area investigated by the Ingolf is bordered Goodfellow (Systematic Part) and Mrs. Agnete to the south by a large unexplored area. Farther Vols4e (General Part). Mrs. Toto §@ling and south, the North Atlantic has been explored by Mrs. Else H~jgaardcarefully prepared the ma- a number of expeditions. The eastern and cen- nuscript for printing and made valuable sugges- tral parts were explored by the Travailleur and tions. the Talisman 1880-1883 (R. Perrier 1902) and Mrs. Edith Asmussen kindly translated Rus- by the expeditions of Prince Albert I of Monaco sian papers for me. with the Princesse Alice and the Hirondelle I1 Finally, I wish to express my sincere thanks (von Marenzeller 1893a, Hdrouard 1902,1923).The to Professor J. Prytz Johansen, former head of Bay of Biscay by the Caudan 1895 (Koehler 1896). the University Library (Scientific and Medical The western North Atlantic was covered by the Department), for allowing me one year's leave Albatross 1893 (Verrill 1885), the Blake 1877- to concentrate on my scientific work. 1880 (ThCel 1886 b), and the Atlantis 1938-1939 During my work I have received invaluable (Deichmann 1940). The Michael Sars crossed economic support from various sources. The the North Atlantic (Grieg 1921). The Swedish University Library paid the expenses of half of Deep-Sea Expedition 1947-1948 made a few my one year's leave, while the Danish Science deep-sea trawlings in the mid-Atlantic, including Foundation paid the other half. The visits to one at hadal depth (7625-7900 m) in the Puerto foreign museums were paid by the University of Rico Trench (Madsen 1953). Copenhagen and the Rask-Orsted Foundation. The German Deep-Sea Expedition 1898-1899 with the Valdivia explored the eastern South Atlantic, the Antarctic Ocean, and the Indian History of exploration Ocean (Ludwig & Heding 1935, Heding 1940). Descriptions of deep-sea holothurians are almost The Investigator 1887-1901 explored the Bay exclusively found in the reports from the various of Bengal and the Arabian Sea (Koehler & Vaney deep-sea expeditions. A review of these expedi- 1905). The Siboga 1899-1900 explored the Indo- tions and of the work dealing with their collec- nesian Seas (Sluiter 1901 b). tions of holothurians, therefore, is at the same A number of expeditions with the Albatross time a history of exploration of this group. explored regions of the Pacific Ocean; most of The Challenger Expedition 1872-1876 foun- them were working close to the coasts: West ded the knowledge of the deep-sea holothurians, coast of Central America 1891 (Ludwig 1894); Hawaiian Islands 1902 (Fisher 1907); North Paci- Only one soft-bottom trawling-station at depths fic coast of North America 1903 (Edwards 1907); exceeding 1000 m failed to bring up holothuri- Japan 1906 (Ohshima 1915, 1916-1919), and ans: St. 302 in the Bay of Bengal (depth: 1190 m, Baja California 1911 (Clark 1913, 1923 a). Only bottom: clay). The station yielded 51 bivalves the expeditions 1899-1900 and 1903-1905 to the and some other benthic invertebrates. eastern Tropical Pacific had a number of sta- In addition to the exploration of the trenches, tions far from the coast. The holothurians from the Galathea concentrated on the exploration the two last-mentioned expeditions were treated of the abyssal fauna, while the bathyal fauna by H. L. Clark (1920). was investigated to a more limited