The amoebae belong to the class Sar- 

 codasida. Members of this class move by 

 means of pseudopods. They have no cilia 

 and, except in rare instances, no flagella. 

 The group is named for sarcode, a term 

 introduced by Dujardin for what was later 

 called protoplasm. Most sarcodasids are 

 holozoic, ingesting their prey by means 

 of their pseudopods. Their cytoplasm is 

 usually divided into endoplasm, containing 

 the food vacuoles, nucleus, etc., and rel- 

 atively clear ectoplasm. The fresh water 

 forms contain one or more contractile 

 vacuoles; these are absent in the salt water 

 and parasitic species. With a few excep- 

 tions, reproduction is asexual, by binary 

 fission or rarely by multiple fission, by 

 budding or by plasmotomy. Most species 

 form cysts. 



The Sarcodasida originated from the 

 Mastigasida. The group did not arise 

 from a single progenitor, but is polyphyle- 

 tic. One line, for example, passes from 

 Tetramitus thru Naegleria to Vahlkampfia. 

 In Tetramitus, which is usually classified 

 among the flagellates, the life cycle in- 

 cludes flagellate and amoeboid stages, 

 and the flagellate stage has a permanent 

 cytostome. In Naegleria, which is usually 

 classified among the amoebae, the life 

 cycle also includes flagellate and amoeboid 

 stages, but there is no permanent cyto- 

 stome. In Vahlkampfia, there is no fla- 

 gellate stage, but the amoebae are very 

 similar to those of Naegleria. Another 

 line passes from the amoeboid flagellate, 

 Histomonas, to the very similar but non- 

 flagellate amoeba, Dientamoeba. 



Only a few of the Sarcodasida are 

 parasitic. The free-living forms include 

 the most beautiful protozoa of all, the 

 pelagic Radiolaria with their delicate, 

 latticework siliceous skeletons. One 

 group of Radiolaria has skeletons of stron- 

 tium sulfate- -perhaps some day proto- 

 zoologists will be asked to develop ways 

 of using them to eliminate strontium 90 

 pollution. Another marine group, the 

 Foraminifera, has calcareous shells. 



Chapter 7 



THE 

 AMOEBAE 



129 - 



