168 



MARINE ANIMALS 



tropics, where the reef corals build gigantic masses of rock. Tropical 

 mollusks often have shells of great thickness ; the snail Magilus, living 

 among coral reefs (Fig. 15), as its shell lengthens with increasing age, 

 fills a large part of it with lime. The shells of the giant clam Tridacna 

 gigas may reach a weight of more than 250 kg. In very cold waters, 

 the calcium in the sea water is inaccessible to the marine animals, and 

 the tropical forms with strong shells or skeletons of this material are 

 accordingly often replaced in polar seas by naked or soft forms, with- 

 out shell or with a delicate chitinous armor, as among Coccolitho- 

 phoridae, Foraminifera, and Pteropoda. The serpulid worms which 

 build lime tubes also reach their highest development in tropical seas, 



r ' r *fc 





Fig. 15. — Magilus antiquus with the coral limestone cuti away to show the shell, 

 which is filled with calcium carbonate to the dotted line. 



and have few representatives in polar waters. Animals which deposit 

 large amounts of lime are also absent in deep seas; serpulids, for 

 example, are wanting, and the deep-sea sea urchins are forms with a 

 soft shell (Echinothuridae). This constitutes an important factor in 

 the distribution of marine animals. 6 Murray 7 has pointed out that the 

 calcium which is brought into the ocean by the rivers, and which comes 

 from the decomposition of the continental rocks, is thus deposited 

 more and more in the tropics at the present time. 



The presence of the inorganic substances of primary importance 

 to plant life, especially those which are not present in excess, such as 

 carbon dioxide, phosphoric acid, and the nitrogen compounds, deter- 

 mines the quantitative distribution of plant life in the sea, and thus 

 secondarily influences the distribution of animal life. 



Sea water contains 40 to 50 mg. carbon dioxide per liter, of which 

 only a few tenths of a cubic centimeter per liter is present in simple 

 solution; 8 it is combined as carbonate or bicarbonate, on account of 

 the excess of basic compounds. Experiments have made it certain that 

 plants are able to use the carbon dioxide contained in bicarbonates. 



