158 MARINE ANIMALS 



belong in this category. The oyster, Ostrea edulis, which endures 

 temperatures from —2° to -4-20° on the Holstein Bank without harm, 

 is eurythermal, and even more so the barnacles like Balanus balanoides, 

 which may be exposed to temperatures below freezing at one time and 

 to the direct rays of the summer sun at another, if attached to rocks 

 near the tide limit. The bivalve Cardium edide and the lugworm 

 Arenicola are others. Eurythermy and stenothermy, of course, inter- 

 grade with one another. Nothing is thus far known as to what may 

 be the physical basis of these variations in animals. 



The greater temperature differences in the sea obviously form 

 effective barriers to distribution. Thus the faunae of the west and east 

 coasts of Florida have a very different composition. 25 The Gulf Stream, 

 which washes the west coast, gives the fauna a tropical character, 

 while on the east coast, at least as far as Cape Canaveral, there is a 

 cold counter current which brings the Carolinian fauna southward. 

 Of 314 species of mollusks, only 145 are common to the two sides of 

 the peninsula; 111 are confined to the west coast, and 58 to the east. 

 Similar differences exist between the west and east sides of Spitz- 

 bergen; the west coast is reached by outlying parts of the Gulf Stream, 

 and the character of its fish fauna, for example, is Atlantic ; the much 

 lower temperatures of the eastern coasts bring with them an arctic 

 faunal character. 20 The Dogger Bank in the North Sea forms an 

 important temperature barrier; north of it the difference between sur- 

 face and bottom temperatures is 7.7°, while to the south, where the 

 cold northern water is kept out, the difference is only 0.8°. The fauna 

 of the northern side of the bank is thus markedly boreal, while to the 

 south it is Atlantic; out of 167 crustaceans on the north side, 85 are 

 absent on the south, and of 97 on the south side, 15 fail to reach the 

 northeastern part of the North Sea. 27 Finally, we may mention the 

 abrupt temperature barrier formed by the submarine Wyville-Thomp- 

 son Ridge, between the Shetland and Faeroes Islands. 28 Northeast of 

 this ridge the temperature at 1100-m. depth is —0.41°, while at the 

 same depth, scarcely a degree further to the south, on the other side 

 of the ridge, the temperature is -4-8.07° (Fig. 14). Accordingly two 

 entirely distinct faunae are brought into immediate contact, the arctic 

 to the northeast of the ridge and southern forms to the southwest. 

 Out of 385 species of animals in this area only 48, or 12.5%, are com- 

 mon to the two sides of the ridge. Temperature differences between 

 bottom and surface at the equator, a distance of six miles or less, are 

 much greater than those between two points on the bottom 10,000 

 miles apart. 



Marine animals present a number of correlations of structure with 



