14 FRESH-WATER RHIZOPODS OF NORTH AMERICA. 



of silex contained in the sea-water. After death, the skeletons of the Radio- 

 laria sink to the bottom of the ocean, where they accumulate as an abun- 

 dant component of the mud. 



In the island of Barbadoes, extensive rock strata of the Tertiary period, 

 1100 feet in thickness, consisting of marls, tripoli, and ferruginous sand- 

 stones, are largely composed of the siliceous skeletons of Radiolaria. 

 Material from these strata called 'Barbadoes earth' is well known to 

 microscopists, and is highly jjrized for the perfection and beauty of the 

 forms it supplies. Like-wise, in the Nicobar Islands, of the Indian Archi- 

 pelago, the solid nucleus of the islands, consisting of clays, marls and 

 ai-enaceous marls of Tertiary age, 2000 feet in thickness, is largely com- 

 posed of the remains of Radiolaria. 



According to Haeckel, the soft body of the Radiolaria is more highly 

 organized than that of the Foraminifera and Heliozoa. It contains a cen- 

 tral capsule of firm membrane enclosing masses of minute cells. The 

 exterior protoplasm commonly contains numerous yellow cells enclosing 

 starch-grains, and in some forms also lai-ge vacuoles, and from it emanate 

 in all directions countless pseudopodal rajs. 



Most Radiolaria possess a highly complex skeleton composed of 

 silex, exhibiting in different kinds a wonderful variety of the strangest and 

 most elegant forms. Sometimes it consists of a simple trellised ball, some- 

 times a series of several such balls enclosed concentrically in one another, 

 and connected together by radial bars. Generally delicate spines, often 

 branching, radiate from the surface of the balls. In other instances, the 

 skeleton consists of a star mostly composed of twenty spines, arranged in 

 definite order and united in a common centre. In some Radiolaria, the 

 skeleton is a delicate, many-chambered shell, as in the Foraminifera. 

 Indeed, says Prof. Haeckel, no other group of organisms develop in 

 the construction of their skeleton such a variety of fundamental forms, 

 with such geometrical regularity, and such elegant architecture. 



The Foraniisiifera {foramen, an aperture; fero, to bear) constitute 

 by far the most important order of the Rhizopods, especially from the vast 

 quantities in which they have existed in all times from the earliest known 

 appearance of life on earth until now, and from the enormous extent in 

 which their remains have contributed to the formation of rocks. They are 

 marine shell-bearing animals, mostly living at the bottom of oceans and 



