PHYSICAL CONDITIONS IN THE OCEAN 153 



use of this means of protection. 10 Corals in moving water tend toward 

 a rounded or flattened and thick mass, with the single polyps closely 

 attached to the upper side. In quiet waters the stalks are lightly calci- 

 fied and finely branched. 11 The delicate Alcyonaria (Spongodes) , on 

 the contrary, in lesser depths where the wave action becomes effective 

 at low tide, are elastic and wave back and forth like grain fields in the 

 wind ; in greater depths, where the water is quiet, they have stiff skele- 

 tons. 12 The hydroid polyp Bougainvillia ramosa, in deeper water, is 

 delicate, slender, and branching. In moving water the stalks become 

 stouter and were formerly supposed to represent a distinct species until 

 the supposed B. fruticosa was experimentally transformed into 

 ramosa. 13 Similar transformations have been demonstrated in Cam- 

 panularia Integra and Sertularella tricuspidata. 1 * 



The continuous currents in contrast with the oscillating motion of 

 the waves and tides are of the greatest importance for the distribution 

 of marine animals. Currents carry characteristic forms of the cold- 

 water plankton, such as the appendiculates Oikopleura labradoriensis 

 and Frittilaria borealis, from the Arctic Ocean into the North Sea in 

 the spring, and in summer they carry warm-water forms like the 

 siphonophore Physophora hydrostatica as far north as the Lofoten 

 Islands. The free-swimming larvae of many fixed or slow-moving ani- 

 mals, like the hydroids and echinoderms, are carried great distances 

 by currents. Thus the gorgonid-fauna of the Bermudas is almost com- 

 pletely West Indian, the larvae being brought to the Bermudas by the 

 Gulf Stream. 15 Occasional surface currents, caused by continued winds, 

 may also carry animals away in definite directions. Thus in the Bay 

 of Naples, where the surface fauna after the Sirocco is especially rich, 

 it is much impoverished after continued north winds, which drive the 

 surface water out of the bay. The jellyfish, Aurelia aurita, sometimes 

 is blown into enormous masses in the river mouths of the south shores 

 of the Baltic by the summer winds. 



The ocean currents are indirectly of extreme importance for the 

 marine fauna. In the first place, they produce the general mixing of the 

 water and maintain the average chemical composition of the sea water 

 at a uniform level, with relatively small local variations. The wave 

 motion favors the solution of oxygen from the air, by increase of the 

 surface area, especially in the crests of waves and in surf and spray. 

 The motions of the water are not merely superficial, but extend to con- 

 siderable depths. Wave motion in the open sea extends to depths of 

 several hundred meters; it reaches a lesser depth near the coast, not 

 below 40 m. on the Algerian Coast; and 200 m. or the "mud line" is 

 usually given as marking the extreme lower extent of wave action near 



