PECULIAR FORMS OF MUSCLE 



103 



in the stalk is a product of the thin layer of cytoplasm that surrounds 

 and nourishes it. 



The fibril is not like the ordinary muscle-fibril in shape and mode of 

 action. It is very large and heavy, and when it contracts it does so by 

 contracting one of its sides. As, according to Entz, this side forms a 



mus. 



mus. c. 



FIG. 102. Muscle cell of Cercaria from Helix, mus.c., muscle cell ; ntus.fi., muscle fibrils. 



(After BETTENDORE.) 



spiral band around the fibril, the result is that the fibril is thrown into 

 a spiral shape resembling a wire bed spring. Our specimens, and con- 

 sequently the illustration, did not show this spiral band. 



The cilia and flagella found on many cells all through the animal 



FIG. 103. Proximal portions of two Vorticella showing the insertion and upper parts of the 



contractile stalks. X 800. 



series, are one of the most primitive forms of motion that affect the cell's 

 relations with the exterior (see other figures). 



These organs are probably passive rods of various lengths and thick- 

 nesses which are moved by the cytoplasm in or near the edge of the cell. 

 They are, apparently, continuations of fibril-like areas inside the cell. 

 This last connection has been used to liken the cell fibrils of the cilia to 

 the rays of the centrosome, which are also organs of motion, and to come 

 back to the centrosome as the fundamental cell-organ of motion. 



