AMEBOID MOVEMENT 93 



the power of contractility highly developed, for the pseudopods 

 readily move about in the water like a tentacle. The demarcation 

 line between ectoplasm and endoplasm is very difficult to see, 

 consequently no definite idea can be given as to the thickness of 

 the ectoplasm. When a pseudopod is being extended the whole 

 contents seem to move at about the same rate as the pseudopod 

 advances, differing thus from amebas, in the pseudopods of 

 which the central core of the endoplasmic stream flows consider- 

 ably faster than the tip of the pseudopod advances through the 

 water. But when a large pseudopod is cut off from a Difflugia 

 it is able to move after the manner of an ameba without a nucleus 

 (Verworn, '94). 



In heliozoans protoplasmic streaming is quite different from 

 that in ameba or Difflugia. The pseudopods are usually straight, 

 radiating from the central body. They possess usually a central 

 axial rod of condensed or strongly gelatinized protoplasm around 

 which is a layer of thick protoplasm with the properties of ecto- 

 plasm. Heliozoans for the most part move slowly ; in fact many 

 of them are pelagic and in these the power of locomotion on a 

 solid substratum is very slow. There is however one species, 

 Acanthocystis ludibunda, which, according to Penard ('04), can 

 move twenty times its diameter in one minute by rolling. This 

 illustrates a highly developed power of contractility in the pseudo- 

 pods of this organism, for since only about one-fifth of the cir- 

 cumference can be in contact with the solid substratum, the pseu- 

 dopods must attach themselves, contract so as to pull the Acan- 

 thocystis along, and relax their hold, all in the space of two 

 seconds. 



Among pseudopod forming organisms, the highest develop- 

 ment of contractility is found in the foraminifera. As is well 

 known, these organisms form finely anastomosing pseudopods 

 which frequently cover the substratum with a network of proto- 

 plasmic strands. The terminal sections of these strands are fre- 

 quently so thin and transparent that they cannot be seen easily 

 with the microscope. As a rule the granular endoplasm is ob- 

 servable only in the main body of the organism and in the larger 

 trunks of the pseudopods. Much the larger part of the pseudo- 

 pods, as measured lineally, is devoid of granular endoplasm. The 



