3 o AMEBOID MOVEMENT 



bodies (Stole, 'oo) which fill it to capacity, lie so close to the sur- 

 face that it is frequently impossible to see any protoplasm between 

 them and the exterior. The contractile vacuoles which are nu- 

 merous, also testify in their characteristics, to the ease with which 

 the ectoplasm may be broken. The vacuoles never reach but a 

 very small size (four microns in diameter) presumably because of 

 the thin consistency of the endoplasm and because they can 

 readily break through the ectoplasm. They burst on the surface 

 of the ameba instantaneously, as a small air bubble might burst 

 on pure water. But this ameba differs from limicola in that a 

 cross section of the body is very nearly a circle. 



Another very interesting feature of Pclomyxa, schiedti is the 

 uroid (Figure 6, ), which in this species consists of a number 

 of very thin projections resembling pseudopods extending from 

 the posterior end. These projections are attached to the sub- 

 stratum and in some way aid in locomotion. These uroidal pro- 

 jections are of considerable length, and may persist for a consid- 

 erable length of time. Thus while schiedti is unable to form 

 pseudopods at its anterior end, it forms uroidal projections with 



Figure 7- Amoeba radiosa, after Penard. a, the rayed stage, b, the 

 rayed stage in which some of the pseudopods are being withdrawn. One 

 of them is thrown into a spiral as it is being withdrawn, c, the stage pre- 

 ceding the trophic stage shown at d. 



