LIFE IN THE OCEAN CLARK 389 



and local, which eats largely, if not mainly, sea urchins. The sea 

 snakes, true snakes and poisonous, yet true sea animals, most of 

 them more helpless on land than eels, the seaweed eating iguanas of 

 the Galapagos, and the coypu of the inlets of southwestern South 

 America also deserve mention. But the most curious of all the 

 seacoast creatures is the large fish-eating bat of the Caribbean re- 

 gion, which smells strongly of musky fish oil and is abundant at 

 St. Vincent, where it spends the day in chinks and crevices in the 

 sea cliffs which one would think much too small for it. 



What is the vegetable basis of this abundant coastal life? 



On our north Atlantic coasts and on the coasts of Europe this 

 comes from four main sources. 



1. Vegetable detritus, or the more or less decayed fragments of 

 the plants growing on the bottom, the seaweeds and the eelgrass, 

 suspended or dissolved in the water, lying on the bottom, or mixed 

 with the bottom mud. 



According to very careful investigations which have been carried 

 on in Denmark all the bivalve mollusks, two snails, all the sea 

 cucumbers, sipunculids, cumaceans, sea squirts, ostracods, polyzoans, 

 sponges, and foraminifera, and the balanoglossids and cephalochor- 

 dates, as well as the beach-fly larvae, are purely detritus feeders; 

 the great mass of material in their alimentary tracts when analyzed 

 corresponds to the detritus on the ocean floor, and the free-floating 

 plants are only incidentally present. In the deepest water the 

 organic matter is probably chiefly derived from the free swimming 

 organisms which die and fall to the bottom, and in places from 

 the Sargassum and other rock weeds which have been torn from 

 their moorings and have perished far from land. 



2. Plants growing on the bottom, chiefly eelgrass where that occurs, 

 upon which browse certain snails, like the periwinkles, a few echino- 

 derms, and some crustaceans. The Danish naturalists have found 

 that as a basis for the support of the shore-living animals these 

 plants (especially the eelgrass, the most abundant on the Danish 

 coasts) are next in importance to detritus. 



3. Free swimming microscopic plants, similar to those of the open 

 ocean. The Danes have found that these are of almost no importance 

 on their coasts; their slight value is indirect, through the medium 

 of the free swimming copepods. But probably elsewhere, especially 

 in Arctic and Antarctic regions where there is no eelgrass and they 

 are enormously abundant, they become of much significance. 



4. Driftwood, floating or stranded in the water, and wooden struc- 

 tures, such as piles and wharves. These, essentially vegetable 

 detritus, form the food of curious aberrant bivalves called ship- 

 worms or teredos, which bore into them and often cause enormous 

 damage. Other bivalves and various crustaceans, such as the gribble, 



