IIO PROTOZOA. 



Class III. — Infusoria (Infusorian Animalcules). — Protozoa which 

 are typically provided with a mouth and rudimentary digestive cavity, 

 which do not possess the power of emitting pseudopodia, but which are 

 furnished with vibratile cilia or with contractile filaments. They are 

 mostly microscopic in size, and are mostly destitute of the power of se- 

 creting hard skeletal structures. 



Regarded palseontologically, we may eliminate from the Protozoa 

 the entire class of the Gregarinidce, with the Rhizopodous orders of 

 the Monera 1 and Amoebea, 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 Animal- 

 cules. 2 Some of these, however, possess chitinous 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 

 PeiHdinium and allied forms of Flagellate Infusoria. With this 

 doubtful exception, however, no Infusorian animalcule has ever been 

 detected in the fossil state, though the class has doubtless existed 

 from the most remote antiquity. There remain, then, only the 

 three Rhizopodous orders of the Foraminifera, Radiolaria, and Heli- 

 ozoa, in all of which the soft protoplasmic body is generally strength- 

 ened by hard structures of horn, lime, or flint. The last mentioned 

 of these orders comprises fresh-water organisms, in which a skeleton 

 is absent or imperfectly developed, and no traces of fossil Heliozoa 

 have hitherto been met with. On the other hand, the Foraminifera 

 and Radiolaria usually have a well-developed skeleton, and are 

 more or less extensively represented as fossils, so that they demand 

 attention separately and in detail. 



Foraminifera. 



The Foraminifera may be defined as Rhizopoda 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 body- 

 substance gives out long and thread-like processes (pseudopodia), which 

 interlace tvith one another to form a ?ietivork, and often coalesce at 

 their bases to form a continuous layer of sarcode outside the shell. 

 The pseudopodia (fig. 18) reach the exterior either by perforations in 



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

 but they will be considered here as belonging to the vegetable kingdom, and they 

 will be briefly described in speaking of fossil Alga. 



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

 posits in the earth's crust, but the organisms so named are really Diatoms and 

 Polycystina, 



