DISTRIBUTION OF INDO-PACIFIC LITTORAL HOLOTHXJRIOIDEA 419 



are separated by land barrier, Ekman postulated a direct connection 

 in the past between the two regions. This direct connection of the two 

 oceans has been corroborated by paleontological and geological evidences. 

 It has been shown that for long periods of the Palaeozoic and Mesozoic 

 eras the Pacific had a direct connection with the Atlantic across the 

 present Central America. This was changed in the Tertiary period. 

 Again in the Eocene, Oligocene and Miocene the two oceans had a di- 

 rect connection for considerable period which, according to Schuchert, 

 has existed without interruption. This accounts for the many identical 

 forms of holothurians and other marine life in the two oceans. 



The holothurians common to both tropical waters of the two oceans 

 are said to be circumtropical. Holothurians recorded from the West 

 Pacific, Central Pacific, East Pacific, Indian Ocean and the Atlantic 

 Ocean are circumtropical in distribution. The occurrence of identical 

 species around the tropical warm waters may be understood if the geo- 

 logical history of the past is reviewed. It is said that across the greater 

 part of our planet an immense sea once stretched, mainly in an easterly 

 and westerly direction, dividing the continents into two main groups, 

 a southern and a northern group. It connected the East Pacific, the 

 Central Atlantic, the Mediterranean, the Indian Ocean and the West 

 Pacific with one another. According to Suess this extensive sea in the 

 remote past was known as "Tethys", the name of the wife of the God 

 Okeanos. Other names given to this sea are Mediterranic, Mesozoic 

 Mediterranean, Numulite Sea, Mesogee, etc. During the whole of the 

 Mesozoic era and the early Tertiary Period, the Tethys Sea was of con- 

 siderable size. The Indo-West Pacific, the Mediterranean, the tropical 

 Atlantic and East Pacific faunas were, therefore, parts of one major unit, 

 the Tethys fauna. To understand the present warm-water fauna there- 

 fore, it is of great importance to know the former Tethys fauna thru 

 paleontological studies. 



One of the most interesting facts is that the early Tertiary Atlantic 

 fauna had a distinct Indo-West-Pacific character similar particularly to 

 that of the Mediterranean fauna. Many groups of marine animals now 

 found confined to the Indo-West-Pacific have also been found to have 

 existed in the East Atlantic Ocean. Although there was no holothurian 

 reported among the echinoderms because they do not usually fossilize, 

 the present holothurian fauna shows several of them to be circumtrop- 

 ical. We may cite a couple of them to illustrate the existence of 

 identical species around the tropical waters of both oceans to give light 

 to the existence of the Tethys Sea in the remote past. Holothuria imi- 

 tans Ludwig and H. difficilis Semper are among the several circum- 

 tropical species listed. Their occurrence, particularly on both sides of 



