488 DISTRIBUTION OF LIFE IN THE OCEAN 



other flotation devices, others gradually sink and finally 

 settle to the sea bottom where they form extensive de- 

 posits. It has been calculated that in certain districts 

 this sediment increases one inch every ten years, and 

 the number of organisms included is beyond compre- 

 hension. In England and France, for example, there 

 are great chalk deposits, approximately 600 feet in thick- 

 ness, and in this country similar accumulations of some- 

 what lesser thickness extend through several states. This 

 material, in large measure, consists of the skeletons of 

 unicellular organisms that number from 1,000,000 to 

 4,000.000 to the cubic inch. 



Some Pelagic many-celled Animals. — In addition 

 to the free-swimming or floating young of practically 

 every large group of the animal kingdom, there are many 

 achilt animals of widely different form and relation- 

 ship which also swim or float (Fig. 141). The most 

 important of these are the copepods, small shrimp- 

 like organisms bearing a slight resemblance to a minia- 

 ture mandolin. These creatures, rarely over a fourth 

 of an inch in length, feed on plants, and in many regions 

 are so abundant that they are practically the only food 

 of many commercially valuable fishes as well as of sev- 

 eral species of whales. They are also preyed upon by 

 shoals of jelly-fishes, various worms, several species of 

 free-swimming snails and numerous other creatures re- 

 lated to the vertebrates. Many other animals, such as 

 the squids, fishes large and small, shrimps and crab-like 

 animals of various forms and sizes usually with crimson 

 colored bodies or as transparent as glass, also live upon 

 copepods or upon other animals that feed upon them. 

 At the same time it should be noted that very many 

 other animals feed upon plants, but none of these is so 

 widely distributed or of such high economic importance 

 as the copepod. 



Organs of Flotation. — Protoplasm is somewhat 

 heavier than sea water, and hence will sink. To over- 



