PROTOZOA 



225 



510. In the order Am&bina, to which amoeba belongs, there is no fixed 

 form of body; there is no membrane, shell or skeleton of any kind. Repre- 

 sentatives of this order are found in both fresh and salt waters and many 

 are parasitic. Entamoeba coli is a common harmless parasite in the 

 human intestine and Entamoeba histolytica is the cause of tropical 

 dysentery. The order Heliozoa comprises rhizopods which have a spher- 

 ical central body from which radiate numerous long, slender, ray-like 

 pseudopodia. The body may be either naked or surrounded by a gel- 

 atinous or silicious capsule perforated by numerous pores through which 

 the pseudopodia project. Sometimes the cell is attached to other objects 



FIG. 126. Actinomma, a Radiolarian. A, Whole animal with a portion of 

 two shells removed to show the interior. B, section, showing concentric shells, 

 radial spines and central capsule (c) ; n, nucleus; p, protoplasm. (From Galloway, 

 after Parker and Haswell.) 



by a slender stalk. Heliozoa are found in fresh and salt water. They are 

 never parasitic. The order Foramenifera includes fresh- and salt-water 

 rhizopods which have a shell composed of gelatinous or horny matter, to 

 which may be added calcareous or silicious secretions deposited by the 

 protoplasm, or minute foreign particles like grains of sand. The pseudo- 

 podia may be amceboid in form or long and slender like those of the heliozoa 

 but differing from the latter in the less regular arrangement and constantly 

 changing form. Many species add successively larger chambers to the 

 first shell. These are often in a spiral arrangement. In the order 

 Radiolaria there is found a peculiar structure called the central capsule 

 which encloses the nucleus and the central part of the protoplasm. Out- 



