98 PROTOZOA. 



Eegarded palseontologically, we may eliminate from the 

 Protozoa the entire class of the Gregarinidce, with the Rhizo- 

 podous orders of the Monera l and Amcebea, no trace of the 

 past existence of which has yet been obtained, or, from their 

 soft-bodied nature, is ever likely to be. For all practical 

 purposes the same may be said of the large and universally- 

 distributed class of the Infusorian Animalcules. 2 Some of 

 these, however, possess horny or membranous cases, which 

 might possibly be preserved in a fossil state ; and Ehrenberg 

 has found in the flints of the Chalk certain microscopic 

 bodies, which he regarded as being the protective carapaces 

 of Peridinium and allied forms of Flagellate Infusoria. 

 With this doubtful exception, however, no Infusorian ani- 

 malcule has ever been detected in the fossil state, though 

 the class has doubtless existed^ from the most remote an- 

 tiquity. There remain, then, only the three Rhizopodous 

 orders of the Foraminifera, Radiolaria, and Spongida, all of 

 which secrete hard structures, and all of which are more or 

 less extensively represented as fossils, so that they demand 

 our attention separately and in detail. 



I. FORAMINIFERA. 



The Foraminifera may be denned as Kkizopoda in which 

 the body is protected by a shell or " test," which is composed of 

 carbonate of lime, or which may consist of particles of sand 

 cemented together "by some animal cement, or may be simply 

 horny (chitinous). The animal may be simple, or may repeat 

 itself indefinitely by budding, and the body -substance gives out 

 long and thread-like processes (pseudopodia) which interlace 

 with one, another to form a network, and often coalesce at tlwir 

 to form a continuous layer of sarcode outside the shelL 



1 The "coccoliths" are sometimes regarded as being referable to the 

 Monera ; but they will be considered here as belonging to the vegetable king- 

 dom, and they will be briefly described in speaking of fossil Algae. 



2 " Fossil Infusoria" are often spoken of as forming more or less extensive 

 deposits in the earth's crust, but the organisms so named are really Diatom* 

 and Polycystina, 



