214 FRESH-WATER BIOLOGY 



pseudopodia are usually supported by "axial filaments" which con- 

 sist of stiffened protoplasmic supports forming the axes of the 

 pseudopodia along which the soft protoplasm flows. These axial 

 supports enter the body, the inner ends apparently resting on or 

 near the nuclear membrane. The supports permit the flexing of 

 the rays and at times they may become soft and be absorbed by 

 the protoplasm of the body. 



Shells, which are characteristic of many Sarcodina, may be com- 

 posed of materials secreted by the protoplasm of the animal itself, 

 such as chitin, silica, and calcium carbonate ; or may be constructed 

 of picked-up foreign materials such as sand grains, diatom shells, 

 dirt, etc. 



Shells of chitin are common among fresh-water forms. This 

 material is deposited about the body as a membrane with one or 

 more openings for the pseudopodia. It may or may not be appHed 

 closely to the body and is variable in thickness in different species 

 as well as with age. In young individuals the envelop is usually 

 thin and transparent; with age it may become thicker and more 

 opaque. In many forms the envelop is always thin and flexible, 

 permitting changes in shape of the body from the flowing of the 

 enclosed protoplasm. When the deposit is in sufficient quantity 

 a fimn, rigid shell is produced. If the body does not completely 

 fill the shell the former is united to the inner surface of the latter 

 by protoplasmic threads and is capable of considerable amoeboid 

 movements within the envelop. Some chitinous shells are very 

 delicate, transparent and apparently without separate elements, 

 while others are composed of distinct plates arranged with more 

 or less regularity. 



Species of Difflugia and other related forms are provided with 

 ■shells composed of foreign materials including grains of sand, 

 diatom shells, and particles of dirt. These materials are attached 

 to a thin chitinous layer and cemented together into a compact, 

 rigid shell with one aperture through which the pseudopodia 

 extend. 



Shells may be composed primarily of sihca. In many fresh- 

 water forms these siliceous elements are laid down in the form of 

 regular plates which build up a firm shell. Others, as some fresh- 



