MORPHOLOGY OF ANIMALS 



streaming of semi-fluid protoplasm and is typically illus- 

 trated by the proteus animalcule, Amoeba. In this proto- 

 zoan small lobes, or pseudopodia^ may appear anywhere 

 on the body, and into one or more of these the endoplasm, 

 with all that it contains, may be seen to stream, at the 

 same time being withdrawn from the other lobes. This 

 flowing may continue for some time in a given direction, 

 the outflow of protoplasm at one end of the body being 

 compensated for by the inflow at the other end, thus pro- 

 ducing an actively progressive movement; this is, there- 

 fore, a vortex, the current moving forward through the 

 middle and backward at the periphery (Fig. 9, A). The 

 causes of this movement are obscure, but in some cases 

 it seems to be associated with temporary inequalities in 

 the tension of the surface layer; at points where the ten- 

 sion of this layer is reduced, an outflow of protoplasm 

 occurs, forming a lobe or pseudopod, into which proto- 

 plasm from the main body continues to flow so long as the 

 tension is least at this place. Several points of reduced 

 tension may exist at the same time on the surface of an 

 amoeboid cell, so that several lobes or pseudopodia are 

 found radiating from a common center. In other cases 

 such movement is, perhaps, due to the general contrac- 

 tility of protoplasm, local contraction in one part of a 

 cell causing an outflow in another part. 

 (^) Ciliary movement consists in the rhythmical beat- 

 ing of innumerable small protoplasmic threads {cilia) 

 which project from the free surfaces of certain cells and 

 which act somewhat like oars. Among one-celled organ- 

 isms the entire cell may be covered by these cilia; in all 



Cso;] 



