390 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1923 



bore into wood and are often quite destructive, but the teredo is 

 the only creature known actually to live upon it. 



THE DEEP SEA ANIMALS 



In the open ocean there is abundant plant life at the surface, 

 almost entirely composed of microscopic types upon which feed 

 minute animals endowed with remarkable reproductive powers 

 through which the organic matter is passed on to larger creatures. 

 There are also floating seaweeds torn from the rocks and drifting 

 with the currents, growing more or less, though never fruiting, 

 which serve to some extent to feed the smaller^creatures. The gulf- 

 weed or Sargassum, so common in the north Atlantic, is the most 

 familiar and important type. 



Far below the surface in the twilight zone, where the daylight 

 gradually fades to darkness and plant life disappears, in the levels 

 to which most of the surface animals, at least in the clearer and more 

 sunlit portions of the seas, retreat during the day, lurk many pre- 

 daceous forms which never rise above it. Still farther down, in the 

 cold perpetual night where the motion of the waves is never felt, 

 the creatures of the twilight zone pass over into other types, all of 

 medium size or rather small, all good swimmers, and all or nearly 

 all with phosphorescent lights, like fireflies — strange fishes, squids, 

 crustaceans, jellyfishes, etc. — which, becoming fewer and fewer, 

 reach probably to the bottom, those of each level feeding upon the 

 animals from the zone above, and all being supported by the crea- 

 tures of the twilight zone which at night feed upon the plants. In 

 the north and in the south where the cold water, filled with living 

 particles, is less transparent, and the sun's rays strike it at an angle 

 and do not penetrate so deeply, the twilight zone comes almost to 

 the surface and there is little difference between night and day 

 conditions. 



Along the shores there is a greater or lesser abundance of large 

 algse or seaweeds of many sorts and of flowering plants living 

 fastened to the bottom. These are constantly dying and, partially 

 decaying, breaking up into fine particles, this detritus floating about 

 in the water and finally coming to rest in the mud or sand. The 

 microscopic plants of the open ocean, of course, exist here also, while 

 many kinds of diatoms and similar types live on the bottom and 

 clinging to the weeds. While a few animals here live by browsing 

 on the seaweeds and the eelgrass, the dominant animal types are 

 sluggish or sessile, or attached and usually arborescent plantlike 

 animals, living on the bottom or traveling over it, consuming the 

 detritus, with the more active animals consuming them, especially 

 the shellfish, crustaceans, and echinoderms. 



What happens as the sea floor sinks farther and farther from the 

 surface ? 



