16 MANUAL OF ANIMAL BIOLOGY 



tinuous and intense activity undoubtedly would be revealed. An 

 indication of this is to be seen in the physical phenomenon known 

 as brownian movement. This occurs, for example, when a finely 

 ground, insoluble substance is placed in water. A microscopic 

 examination reveals a continuous, spontaneous movement of the 

 particles, which is believed to be the expression of the activity of 

 the ultramicroscopic molecules in the liquid. 



Although, then, basically, this phenomenon cannot be regarded 

 as a unique feature of living organisms, visible spontaneous move- 

 ment is, nevertheless, the most striking and readily recognized 

 indicator of life. By means of it organisms are able to perform 

 various life functions. Thus movement is closely associated with 

 nutrition in many forms of life. If the food supply is scarce or 

 other environmental factors are unsuitable, many types of organ- 

 isms are so equipped that they can move to a more favorable 

 region. Indeed movement is vitally important in almost every 

 phase of life ; a fact that can probably be shown to the best advan- 

 tage by noting a few representative types of protoplasmic move- 

 ment. 



Flowing Movement. In many types of plant and animal cells 

 a flowing, or streaming, of the protoplasm is easily observed under 

 the microscope. In a unicellular form like the Amoeba where the 

 protoplasm is not enclosed in a rigid cell wall, the direction and 

 extent of the protoplasmic flow is irregular and depends largely 

 upon the environmental factors. In higher types of cells with 

 more or less rigid cell walls a regular streaming, or cyclosis, of 

 the enclosed cytoplasm occurs. This can be seen to good advan- 

 tage in a unicellular animal like Paramecium. An especially good 

 example is to be seen in the leaf cells of the fresh-water plant 

 Elodea. In an active cell, a microscopic examination will show 

 that the disc-shaped chloroplastids are moving around and around 

 just inside the cell wall. This is due to the streaming of the cyto- 

 plasm in which the chloroplastids are embedded. Finally, it is 

 probable that a definite and regular protoplasmic streaming occurs 

 as a necessary feature in cell division. It has been found possible 

 to detect a protoplasmic flow during this period. (W. fs. 11, 27.) 



Ciliary Movement. Many types of cells possess cilia. These 

 are extremely fine, protoplasmic filaments which project from the 

 cell surface. They exhibit a beautiful, coordinated lashing move- 

 ment which, in the unicellular forms, serves to propel the organism 



