80 BIOLOGY: GENERAL AND MEDICAL 



There is but one flagellated cell in the higher animals > 

 the spermatozoon, in which the single flagellum, called 

 the tail, propels the cell like the tail of a tadpole and 

 follows the body or head. The purpose of the flagellum 

 is to enable the spermatozoon to ascend the reproduc- 

 tive passages (Fallopian tubes) until it meets the ovum 

 for fertilization. 



Contractile Movements. Certain cells undergo a physi- 

 ological specialization, by which they are enabled to con- 

 tract and so produce movements advantageous to the cell. 

 It is difficult to determine exactly where this function 

 begins. It might be attributed to the amoeba and 

 explain the withdrawal of its pseudopodia were it not 

 impossible to find any evidence that these processes in 

 any manner differ from the remainder of the cytoplasm. 



In vorticella we see a pedicle or stalk consisting of an 

 extension of the cytoplasm (ectoplasm) endowed with 

 active contractile powers, and showing a peculiar spiral 

 structure to account for it. 



In many hydras the tentacles are armed with nettle- 

 cells or stinging cells, irritation of which causes the 

 sudden projection of fine stinging cells by which the 

 prey of the animal is paralyzed. When the stinging 

 function is performed, the hair-like process is again 

 withdrawn through a contractile specialization of that 

 part of the cell. 



As the scale of animal complexity is ascended and 

 increase of size and differentiation of structure becomes 

 more and more marked, the necessity for special mechan- 

 isms by which the necessary movements for carrying 

 on the functions is more imperatively experienced, 

 special cells and groups of cells become endowed with a 

 structure adapting them for contraction. These are 

 known as muscle cells and eventually lose most of their 

 cellular characteristics through the differentiation of their 

 substance into fibrillae composed of alternating discs, 

 chemico-physical disturbances of which result in length- 

 ening or shortening and thus in movement. 



