76 THE SMALLEST LIVING THINGS 



The Sarcodina 



Characteristics 



The Sarcodina type of protozoa is not as definite as the 

 flagellate type, although there are well marked characteristics 

 common to the entire group. The protoplasmic body is rarely 

 restrained by a firm membrane, and streaming movements, 

 whereby portions of the body substance flow outward as pseudo- 

 podia,* are characteristic. There are, however, different types 

 of pseudopodia. Some of them are delicate and needle-like and 

 resemble flagella in having an axial filament and a protoplasmic 

 sheath (axopodia). These are typical of the sun-animalculae 

 (Heliozoa), and of the marine Radiolaria, which comprise the 

 largest group of known protozoa. Another type of pseudopodia, 

 called myxopodia, are much less rigid, and their protoplasm has 

 a remarkable miscibility,'*' such that two or more pseudopodia 

 of the same organism, on coming in contact, fuse to form great 

 nets of protoplasm, which act as food-traps for catching other 

 kinds of minute organisms. This type is represented by the 

 Foraminifera (Fig. 44), another great group of marine pro- 

 tozoa, second in importance only to the Radiolaria. 



Rhizopods 



In fresh water the commonest sarcodina are the rhizopods. 

 These are naked forms such as Ameba, or shell-bearing forms 

 like Arcella, Difflugia, Euglypha, etc. The shells are interesting 

 in that they are frequently composed of minute foreign particles, 

 such as sand crystals, diatom shells, or even of living algae, which 

 are cemented onto the outside of a shell membrane. In all cases 

 this membrane is composed of chitin* or pseudo-chitin, which is 

 derived from the protoplasmic protein. In Arcella it forms the 

 shell without additional plates or particles, but foreign particles 

 as in Difflugia, Centropyxis, etc., or home-made plates and blocks, 

 as in Euglypha, Quadrula, etc., are usually cemented to the out- 

 side. In Foraminifera there are two such chitinous membranes, 



* See the Glossary of this book, page 123. 

 t Miscibility, ability to mix, to blend, to become one. 



t Chitin, a white, horny, crystal-less substance forming the harder part of the 

 outer covering of insects, crustaceans, and other kinds of invertebrates. 



