PSEUDOPODIA AND SKELETON. 183 



nucleus can be further distinguished, which may either remain simple, 

 or, with an increase of size, divide, and that often into a considerable 

 number of nuclei. Again, a varying number of vacuoles may be ob- 

 served, which sometimes exhibit a conspicuous and even rapid con- 

 traction. In the higher forms, the protoplasm becomes differentiated 

 into an outer layer, and a central substance wliich encloses tlie nuclei, 

 and may itself be enclosed by a porous membrane (Eadiolaria). At 

 the same time the body assumes a firmer consistency, and a stable, 

 generally spherical, form, having, however, an appearance varying 

 according to the degree of contraction, and generally showing a state 

 of activity in the pseudopodia, which are protruded as lobose processes, 

 almost as if flowing from the body mass. The form and nature of 

 the pseudopodia vary generally according to the consistency of the 

 protoplasm ; their rigidity and fineness are usually proportional to the 

 firmness of the body-substance. The movement of granules, which is 

 often to be observed in most wonderful fashion on the thin Pseudo- 

 podia, is sufficient to prove that their substance, even when in appar- 

 ent rest, is the seat of a continual molecular displacement. 



We need say the less about the skeleton of the Rhizopoda, since 

 that has only to be considered in the free-living forms, which only 

 interest us here, in so far as they serve more fully to illustrate the 

 general structure of the Ehizopoda. The Ehizopoda can evidently 

 live only in a moist environment, indeed almost wholly in water, but 

 are also found in the earth, and particularly in the thin moist moss 

 or lichen-covered surface of trees and rocks. The skeleton of the 

 fresh-water forms is usually of a simple membranous nature, 

 forming a sort of shell, out of the opening of which the animals pro- 

 trude their Pseudopodia. In tlie marine forms, the skeleton acquires 

 a firmer character by taking in lime or flint, and is frequently of ex- 

 traordinary beauty. This is specially true of the Eadiolaria, which, 

 with their needles and lattice work of flint, are among the most 

 beautiful of microscopic objects. The calcareous skeletons are gene- 

 rally coarser and heavier, and lie quiescent with their occupants among 

 foreign substances ; while the Eadiolaria, with their delicate skeleton 

 and the radiating meshwork of protruded Pseudopodia, are often 

 found floating in the water. Like the chitinous shell, the calcareous 

 skeleton has often only a single opening, but in the majorit}- of cases 

 it is perforated by countless small pores through whicli the Pseudo- 

 podia issue (hence the name " Foraminifera "). The Polythalamia 

 have a number of united chambers, which increase in number with 

 the increasing size of the body. The closed form and iumioveable 

 walls of the house indeed necessitate a new building if the body is 

 to mcrease m size. Digitized by Microsoft® 



