THE LIVING SUBSTANCE. 37 



fluidly to the laws of its structure, the foam structure, and 

 becomes a spherical food sac.^ 



Here we have a physical pellicle of the substance, formed at 

 moment of a splitting apart of the endosarc, modified into a 

 true contractile pellicle. After functioning for a time in this 

 way, it is as a viscid fluid transferred to another portion of the 

 endosarc, and there being for a time relaxed, functions merely 

 passively and permits the passage of fluid through it. Then 

 it may more or less often function again as a contractile 

 membrane during the digestive process. Finally, it is again 

 forced outwards by a contraction wave of the surrounding 

 substance and beneath the peristome collapses, expelling its 

 contents to the exterior, after the manner of a contractile 

 vacuole ; or it may itself also be expelled by the endosarc and 

 perish as waste matter with its contetiis. 



Beside the formation of digestive sacs, these activities of the 

 substance were seen to be made use of to rid the organism 

 of unwelcome presence in the pharyngeal cleft of large num- 

 bers of bacteria which were whirled into it by the cilia. 

 Under similar circumstances many rotifers would simply 

 turn their digestive tract inside out, and so get rid of the 

 intruders. 



Here, with no visible nervous organization, the same result 

 was had in rather a more complicated way ; for the end of the 

 pharynx, after being pinched off, was forced all the way up 

 through the body mass and expelled, walls and contents at 

 once into the water. 



[31] In a single life phenomenon there is here shown a 

 selective power of the substance ; a migratory power as a sub- 

 stance organ ; metamorphosis of both substance structure and 

 substance function ; rhythmic changes of viscosity ; and above 

 all there is shown a correlation of impulse, a coordination of 

 these activities throughout the entire mass, which is as perfect 

 as that seen in organizations, where, to make it more com- 

 prehensible (.'), there are enormously complex systems of struc- 

 tural modifications of the substance. 



^ These appearances were construed by Ehrenberg into a complex oesophageal 

 structure. 



