SOIL PROTOZOA 313 



forms, are microscopic and can be studied in detail only with the highest 

 magnifications. The protoplasm is a colloidal liquid containing chro- 

 matic or nuclear substance, generally forming nuclei readily distinguish- 

 able from the protoplasmic body, which is either naked at the surface 

 or enclosed by a cell membrane. Usually one or two nuclei are present, 

 in some cases several of them; most infusoria contain a large macronu- 

 cleus (vegetative functions and asexual division) and a small micronu- 

 cleus (sexual reproduction). Contractile vacuoles, when present, are 

 for the elimination of waste fluids or possibly for the adjustment of the 

 osmotic pressure of the protoplasm (absent in marine forms). Green, 

 yellow or brown chromatophores are present in the endoplasm of some 

 Mastigophora. The most important constituents of the cell are the 

 complex proteins, particularly nucleins and nucleo-proteins. In addi- 

 tion to these, there are always present in the living cell carbohydrates, 

 lipoids and enzymes. There are also found in the protozoa undigested 

 food particles, waste materials or foreign elements, which take no part in 

 the physiology of the organism; algae and bacteria may often be present 

 in the endoplasm, possibly as a result of symbiotic relationship. Many 

 species are subject to the attacks of minute parasitic organisms, 

 either in the nucleus or in the cytoplasm. 



Reproduction is usually effected by fission, and, in the great majority 

 of protozoa, a process of conjugation occurs at some stage in the life- 

 cycle, the essential part in the process being fusion of the nuclear matter 

 from distinct individuals. Locomotion is accomplished either by cilia, 

 flagella, pseudopodia, or may be absent entirely, this serving as a basis 

 for classification. Organs for the capture and assimilation of food may 

 be present or entirely absent. The protozoa are classified on the basis of 

 locomotion into: 



1. Sarcodina or Rhizopoda, motility by means of pseudopodia, i.e., extensions 

 (usually temporary) of the protoplasm of the cell body. The pseudopodia are 

 either broad, blunt, finger-like or filiform, simple or branched. In some (Helio- 

 zoa), the ray-like pseudopodia are usually supported by axial filaments. Some 

 Sarcodina are naked, while others form shells; these are composed of materials 

 secreted by the animals themselves, as chitin, silica, calcium carbonate, or are 

 constructed from foreign particles, as diatoms, sand grains, clay particles, etc. 

 Some shells (chitinous) are delicate, transparent, while others are composed of 

 distinct plates, arranged more or less regularly. 



2. Mastigophora or Flagellata, motility by means of flagella. These flexible, 

 whip-like processes are usually attached at one end of the body. Either one or 

 more flagella may be present; when single the flagellum is usually directed for- 

 ward and draws the body forward by its movement. When more than one 



