PHYLUM PROTOZOA. ONE-CELLED ANIMALS 



39 



as strong chemicals, heat, and mechanical 

 impacts, carry the animal out of danger. 



The data thus far obtained indicate that 

 factors are present in the behavior of the 

 amoeba "comparable to the habits, reflexes, 

 and automatic activities of higher organ- 

 isms" (Jennings). 



OTHER SARCODINA 



The amoeba was first reported by Roesel 

 in 1755, although what species he saw is in 

 doubt. Our type. Amoeba proteus, has been 

 described as "a shapeless mass of proto- 

 plasm," but this is incorrect. Although it is 

 continually changing its shape, it has definite 

 characteristics such as a disk-shaped nucleus, 

 blunt pseudopodia, and often longitudinal 

 ridges on the surface. Many amoebas from 

 fresh water, salt water, soil, and as parasites 

 in other animals, have been described; some 

 have been placed in the genus Amoeba, and 

 the rest have been assigned to other 

 genera. 



Pelomyxa palustris is a large species that 

 may reach a diameter of 2 mm.; it contains 

 many nuclei and moves along without defi- 

 nite pseudopodia. Another large species, 

 Pelomyxa carolinensis, which is sometimes 

 referred to as Chaos chaos or giant amoeba, 

 may be obtained from biological supply 

 houses. This species is from 50 to 500 times 

 the volume of Amoeba proteus; it may reach 

 a length of from 2 to 5 mm. (^100 to 2%oo 

 inch), and can be seen with the naked eye. 

 Pelomyxa carolinensis usually contains from 

 300 to 400 nuclei, and from 3 to about 12 

 contractile vacuoles. Instead of dividing into 

 2 daughter amoebas, it generally divides into 

 3. Parasitic amoebas are described in a later 

 chapter. 



Several types of common fresh-water Sar- 

 codina are protected by shells. Arcella (Fig. 

 17) secretes its shell, but Difflugia (Fig. 17) 

 builds a shell of minute grains of sand. In 

 both types, pseudopodia are thrust out 

 through a circular opening in the shell; they 



serve, as in Amoeba, for purposes of loco- 

 motion and obtaining food. Another inter- 

 esting fresh-water species is sometimes called 

 the sun animal, Actinophrys (Fig. 17) be- 

 cause of its stiff radiating pseudopodia. This 

 is abundant among aquatic plants. The ray- 

 like pseudopodia are stiff because each con- 

 tains an axial filament to keep it rigid. 



Most of the 8000 or more Sarcodina live 

 in the sea. The Foraminifera, of which Glo- 

 bigerina is an example, construct a perfo- 

 rated shell, usually of calcium carbonate, 

 through which slender pseudopodia project 

 (Fig. 17). Radiolaria also possess slender 

 pseudopodia; many build elaborate skeletons 

 of silica (Fig. 40). 



CLASSIFICATION OF 

 THE SARCODINA 



(For reference purposes only) 



Class Sarcodina includes mostly marine 

 Protozoa which are free-living. They move and 

 capture food by means of pseudopodia. A shell 

 or skeleton may be present. Nutrition is holo- 

 zoic (subsisting on other organisms) and re- 

 production is principally by binary fission. 

 About 8000 species have been described. The 

 two subclasses and four orders are described 

 as follows: 



Subclass 1. Rhizopoda (Gr. rhiza, root; 

 pous, foot). Typically creeping 

 forms with lobose pseudopodia, 

 but no central filament. 



Order 1. Amoebina. Amoebalike. Short, 

 lobose pseudopodia. Some spe- 

 cies (Gymnamoebac: Gr. gym- 

 nos, naked) are naked, whereas 

 other species (Thccamoebae: 

 Gr. theke, case) are covered by 

 a simple shell with one opening. 

 Exs. Amoeba proteus and 

 Arcella vulgaris ( Fig. 17). 



Order 2. Foraminifera (L. forc/mcn, open- 

 ing; fero, bear). With simple 

 or chambered perforated shell 

 and from one to many branched 



