CH. Ill] LIFE IN THE SEA 77 



furnished with long spines and is pierced with holes through 

 which the protoplasmic body of the animal is protruded. Foramin- 

 ifera occur, of course, in the inshore waters but they are dis- 

 tinctively oceanic organisms. 



The Radiolaria are also oceanic planktonic protozoa \Vhich, like 

 the foraminifera, are provided with skeletons ; but the latter, 

 which in the group already discussed are composed of lime, 

 consist of silica. The form of these shells is extremely elegant. 

 Radiolaria are common in seas which contain the normal amount 

 of salt, and therefore are not abundant in inshore waters. In the 

 colder seas they may occur in great numbers, but the greatest 

 variety of species is to be found in the warmer oceans. They 

 occur at all levels in the sea and when they die their siliceous 

 skeletons sink to the bottom and form the well-known Radio- 

 larian ooze. The " fabulous radiolarian treasures " brought home 

 by the Challenger from the bottom of the seas of the globe when 

 worked up by Haeckel led to the discrimination of 4318 different 

 species, and it is quite certain that many more forms of radiolaria 

 remain to be described ^ 



\./ The Diatomaceae are above all the most important organisms 

 in the sea regarded from the point of view of their significance 

 as the producers of organic substance. The diatoms are the 

 '' pastures of the sea " and correspond to the " grass of the fields " 

 of the land. They are quite ubiquitous and are found alike in 

 fresh water, in the sea, at the surface and at the bottom of 

 shallow waters, in mud, sand or on the surface of weeds, &c. They 

 are true plants and are accordingly found only in those parts of 

 the sea into which sunlight penetrates, though the amount of the 

 latter which is sufficient for their metabolism is far less than 

 sunlight in its full intensity. Like the foraminifera and radiolaria 

 they occur in the sea in such incredible numbers as to form 

 immense deep-sea deposits by the accumulation of their dead 

 shells. Diatoms are sometimes called " unicellular Algae." They 

 are vegetable cells containing a nucleus and a variously shaped 



1 Haeckel described 4318 species of Radiolaria. These belonged to 739 genera, 

 which again were grouped into 85 families, and these families were derived from 20 

 orders, while the latter again could be derived from one " common ancestral form," 

 Actissa. It seems "too good to be true." (See Report on Challenger Radiolaria, 



1887.) 



