304 



MOVEMENT IN LOWER ANIMALS 



bundle is sufficient to draw back the whole body of the 

 animal. 



These simple forms of fibrillse give us the key to their 

 character. They are, in fact, merely specially developed 

 portions of protoplasm in which the power of rapid and 

 powerful contraction has been developed to a high rate, 

 leaving the rest of the protoplasm of the cell to perform the 

 ordinary functions of the cell. In the higher forms of 

 animals the cells which contain these fibrillge are almost 

 completely filled by them, and such cells are set aside in 

 the body to perform the action of contraction and expan- 

 sion and produce movement, while the work of digestion, 

 etc., is given to other cells. In 

 general, then, muscle cells are 

 simply cells which have devel- 

 oped to a remarkable extent 

 the modified forms of protoplasm 

 called fibrillse. It nmst be evi- 

 dent, then, that whether we have 

 amcsboid, ciliary, or muscle move- 

 ment, the source of the power is 

 the same, namely the contractile 

 power of protoplasm. In this 

 connection it may be noted that 

 the striated fibrillse have the power 

 of rapid contraction much more 

 highly developed than it is in the 

 non-striated. The enormous ex- 

 tent to which this rapidity of 

 contraction may be developed is seen in the action of 

 the striated wing muscles of the insects (three hundred 

 to four hundred contractions per second in the fly). 



Fi&. 137 — Vorlicella. This 

 protozoan lias a single fiber 

 in the stalk, which by con- 

 tracting draws the animal 

 down as in B. 



