AMOEBOID PROTOZOA (SARCODINA) 213 



flowing protoplasm. In some forms its general position is fixed 

 and it reappears, after contraction, in the same place. As the 

 vacuole becomes inflated by waste fluids and gases it rises toward 

 the periphery and collapses, pouring its contents through the open- 

 ing formed in the body wall. In some of the Hehozoa are seen 

 very large contractile vacuoles which rise to the surface and push 

 the peripheral film outward Hke a bubble before the collapse takes 

 place. 



Many of the shell-bearing forms are capable of raising or lower- 

 ing themselves in the water. This is believed to be brought about 

 by the presence of distinct gas vacuoles. The animals seem to be 

 able to alter the supply of carbon dioxide in these vacuoles and 

 thereby change their specific gravity. 



The ectoplasm, when distinct from the endoplasm, usually ap- 

 pears as a clear hyaline zone, of greater or less width, at the periph- 

 ery of the body. In most forms the vesicles of the ectoplasm 

 are very minute but in some of the Heliozoa they exceed those of 

 the endoplasm in size and may be arranged in a regular manner 

 about the periphery, as in Actinosphaerium eichhornii. 



The protrusion of the ectoplasm is the initial movement in the 

 formation of a pseudopodium after which there may be a flow of 

 the granular endoplasm into the axis of the finger-like extension of 

 the ectoplasm. At times pseudopodia are but broad extensions 

 of the ectoplasm with no appearance of endoplasm taking an\' part 

 in their formation. 



Great variation is seen in the pseudopodia which are character- 

 istic features of the Sarcodina. Among the fresh-water forms 

 several general types may be observed. The naked and many of 

 the shell-bearing Sarcodina produce broad, blunt, finger-like, or 

 more slender, filiform, pseudopodia; the latter may be delicate, 

 pointed and finely branched, but neither of these fuse or anastomose 

 when in contact. Another variety is represented by delicate thread- 

 like pseudopodia which tend to run together and mingle, forming a 

 great network of flowing protoplasm. This is the anastomosing 

 type and is seen in a few fresh-water genera, but is characteristic 

 of many marine forms. 



In the Heliozoa is seen another variety. Here the ray-like 



