80 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



hides resembles the genus Diastylis; P. abbreviata, the arctic Eudorel- 

 lopsis deformis; P. campy laspoides, the genus Campy laspis. 11 



Geographic isolation in the ocean. — The degrees of isolation in 

 the ocean vary in its different parts. The open ocean has few barriers. 

 The continents separate it into several major divisions, but within 

 these the only surface barrier on a geographic scale is that of tempera- 

 ture, whose effectiveness depends upon the eurythermal or steno- 

 thermal character of the animals concerned. Thus the Gulf Stream 

 forms a temperature barrier, which divides the species of craspedote 

 medusae into two groups. 18 Generally speaking, however, the actively 

 swimming pelagic species tend to be world-wide in their distribution. 

 In the tropics the Indian and Pacific faunae agree closely, while the 

 Atlantic differs from both ; the pelagic forms of the Pacific which are 

 absent in the warmer parts of the Atlantic are for the most part 

 merely replaced by allied species. 19 



The conditions are different with the inhabitants of shallow water. 

 Their distribution is limited by more numerous barriers, such as 

 oceanic depths, peninsulas, nature of the bottom, freshening of the 

 water at river mouths, as well as the temperature factors. The ranges 

 of species are consequently smaller, often very small, and the number 

 of species in each genus is larger. Geographic species and subspecies 

 which replace each other in adjoining areas are frequently developed. 

 This applies particularly to forms with slight vagility, which are 

 especially numerous in coastal waters. The sessile actinians, for ex- 

 ample, frequently form endemic species; all the actinians of the New 

 Zealand coast are endemic, and their distribution is dependent on 

 the nature of their larval stages. Those with ciliate larvae are widely 

 distributed, while those which have no free-swimming stage are re- 

 stricted to special areas, in which they may be extremely abundant. 20 

 Of the 10 actinians on the coasts of the Canary Islands, 3 are endemic. 

 Connected marine coasts form highways of distribution for such forms. 

 Of 52 actinian species on the East African coast, 63.1% are endemic; 

 21% occur also in the Red Sea, but only 1.8% are Indian, 5.3% Indo- 

 Pacific. 21 Ascidians are represented on the east and west coasts of 

 northern North America by parallel series, whose members correspond 

 exactly but are distinguishable specifically. 22 



The construction of ship canals at Suez (1869) and Panama (1914) 

 has not opened free channels of communication for marine organisms 

 across these isthmuses to the extent that might be expected on first 

 thought. The Panama Canal is not at sea level, and the fresh water 

 of Gatun Lake kills most animals attached to ships during the several 

 hours required for its passage. The Suez Canal is at sea level; it is 



