THE LIVING SUBSTANCE 



15 



find irregular masses of clear substance which throw out 



tongues in a peculiar way and flow first in one direction and 



then in another. The simplest of all animals, the proteus 



animalcule Amoeba, consists entirely of a small lump of this 



substance, which moves by flowing out in tongues in the way 



described. These movements are consequently termed 



amoeboid, but they are however of fundamentally the same 



character as the contraction of muscle fibres, since the flowing 



.i due to the contraction of one part squeezing out the rest. 



We often find also, both in simple and complex animals, 



projecting hair-like structures termed cilia or flagella, which 



vibrate rhythmically and 



propel the whole animal 



long if it is small, and 



.ropel the surrounding 



duid in a current past 



he animal if the latter 



3 larger. But the vibra- 



ion of these cilia and 



Lagella has also been 



hown to be due to the 



; .Iternate contraction of 



:iarrow slips on each side 



of the vibrating structure; F IG . i._ An Amoeba progressing. The 



.0 that we may sum up direction of movement is towards 



t he matter thus : all the ri ght ; n, the nucleus. 



{ tnimal movement is due 



o contraction in some portion of the moving parts whereby 

 ihat portion becomes shorter and thicker. 



If we could thoroughly explain how this contraction is 

 rought about, we should have penetrated much more deeply 

 into the mystery of life than it has so far been possible to do. 

 But although a complete explanation is not yet forthcoming, 

 ertain broad facts in connection with moving parts may be 

 mphasized The clear substance which is the seat of con- 

 raction is always surrounded by a skin consisting of a thin 

 lastic membrane. When we have to deal with amoeboid 

 novement, this skin is so thin and delicate that it is constantly 

 >eing burst and then formed again round the clear substance 

 vhich has been extruded through the break. This substance 



