136 THE OCEAN 



only in deep water, as in the case of corals, 

 but like the corals other sponges occur along 

 our shores at low-water mark. 



Many other very interesting forms of ma- 

 rine life which in the tropics are found only in 

 deep water, may be discovered along the shore 

 in shallow water in the north, for the limits of 

 the various zones of under-sea life are greatly 

 influenced by the temperature of the water. 

 No doubt, if we could descend to the moderate 

 depths of the ocean in the north we should 

 find the bottom covered with a dense forest 

 of seaweeds, sponges, gorgonias, corals and 

 other marine life, for we know, from the great 

 numbers of specimens that are sometimes 

 drawn up in a single haul of a trawl or 

 dredge, that in places the bottom must be cov- 

 ered by such a living jungle. The only trou- 

 ble is that in northern waters such places are 

 in rather deep water and are far beyond the 

 reach of human eyes. In the tropics, on the 

 other hand, the ocean bed teems with mul- 

 titudes of forms of animal and vegetable life 

 within a few feet of the surface, or even be- 

 tween high and low-water mark, and more- 



