CHAPTER III 



THE SARCODINA 



THE term Sarcodina, introduced by Biitschli ('83) as the class 

 name of the most primitive of the Protozoa, includes all forms which, 

 like the common fresh-water type Amoeba proteus, move by the pro- 

 trusion of protoplasm in the form of broad and finger-like, or sharp 

 and ray-like, processes called pseudopodia. These forms fall naturally 

 into three groups readily distinguished by clearly marked differences 

 in structure, the Rhizopoda, Heliozoa, and Radiolaria. 



Among the Rhizopoda are included forms of Sarcodina with blunt, 

 finger-form or lobose pseudopodia (Amcebida) or with branching and 

 anastomosing pseudopodia (Reticulariida). They may be naked 

 (Gymnamcebina), or shelled (Thecamcebina or Foraminifera). The 

 pseudopodia may arise from all parts of the body or they may be 

 limited to special regions ; in shelled forms they may pass through 

 one common opening {Reticulariida imperforina], or through many 

 finer openings {Reticulariida perforina}. The body form is typically 

 globular, but may be variable in consequence of amoeboid changes, or 

 drawn out into a monaxonic form. The material of the shell may be 

 chitin, silica, foreign particles, or calcium carbonate. 



The Heliozoa are naked or shelled forms of Sarcodina; they are 

 usually globular with fine ray-like pseudopodia arising from all parts 

 of the body. The rays are, as a rule, stiffened by an axial filament 

 formed of modified protoplasm which may be readily dissolved by the 

 organism. The shells are less compact than those of the Rhizopoda, 

 and are usually formed of more or less loosely joined silicious spicules. 



The Radiolaria are similar in form to the Heliozoa. As in the 

 latter, the pseudopodia arise from all parts of the body and occasion- 

 ally anastomose. The endoplasm is separated from the outer plasm 

 by a firm, chitinous, perforated membrane, the central capsule. A 

 test or skeleton, often of exquisite beauty, is usually present, consist- 

 ing of isolated spicules of silica, or of a compact skeleton of acanthin 

 or silica. One or more nuclei are invariably present within the central 

 capsule. 



The finer structure of the rhizopbd protoplasm has already been 

 mentioned. In many cases, especially in the monothalamous forms, 

 the plasm is divided into a number of clearly marked zones. Schewi- 



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