254 MARINE ANIMALS 



derived from the bodies of the surface plants and animals which sink 

 to the bottom, and on the falling excrement of the surface animals. 

 Herbivores are consequently wanting in the deep-sea fauna proper; 

 only scavengers and predators are represented. Food falls like gentle 

 rain into the depths, its abundance depending on the number of animals 

 in the upper zones. The rate of sinking of the food materials is slow. 

 A salpa of 5-cm. length sinks 40 cm. in 20 seconds, at which rate it 

 would require 2 days and 7 hours to reach a depth of 4000 m. The low 

 temperatures of the deeper water prevent decomposition, even in this 

 time. The greater abundance of neritic than of pelagic animals, plus 

 additions brought in by streams and winds from land, produces a 

 greater supply of food in the littoral depths, and hence a greater con- 

 centration of animals, than is to be found in the depths beneath the 

 open ocean. 11 The falling food supply is continually diminished on its 

 way by various animals of the intermediate zones, so that, under equal 

 conditions, the deeper the sea the less of this food supply reaches the 

 bottom in a form available as food for animals. This must be one of 

 the reasons for the reduction of population density with depth. 



The remnant of the falling food materials accumulates on the 

 bottom, where it forms the food supply of the benthic animals of the 

 deep sea. Numerous species of Foraminifera live on it. Many echino- 

 derms have the same feeding habit in the lighted zone, so that their 

 transition to the lightless benthal was simple; holothurians, especially, 

 are the most abundant deep-sea animals. 12 Many snails adopt similar 

 feeding habits, for example, the Trochidae. Their digestive tract is 

 elongated, their radula reduced, and the excretory tube is elongated 

 and carries the excreta away from the feeding ground. The Pleuroto- 

 midae have given up their predaceous habits in the deep sea. 13 

 Amphipods and isopods are already detritus feeders and flourish in 

 the deep sea. Hydroid polyps, sponges, lamellibranchs, worms, and 

 cirripedes live on the detritus, but make it available by water currents 

 instead of feeding on it directly. 



Predaceous deep-sea forms require little comment. Some fishes are 

 characterized by extraordinarily wide mouths and very strong denti- 

 tion, their distensible stomachs sometimes containing prey larger than 

 themselves (Melanocetus, Saccopharynx, Eupharynx) . In general the 

 predaceous animals of the depths display no special characters and no 

 special superiority on account of the greater severity of competition. 



Deep-sea animals are usually small in comparison with their rela- 

 tives at the surface. Lamellibranchs and snails are often minute, and 

 rarely reach moderate size. Fishes are of relatively small length; 

 Chimaera, a meter in length, is large; the shark Scapanorhynchus, 





