MUSCLES 



175 



this there are four large masses which run longitudinally, two 



on the dorsal side and two on the ventral. 

 410. The muscle systems and the integument form a hollow 



cylinder, which is closed at both ends and filled with a fluid, 



the body fluid. This constitutes a locomotor mechanism, 

 which operates as follows: When the 

 circular muscles contract the body be- 

 comes more slender, and must, there- 

 fore, elongate, which causes the longi- 

 tudinal muscles to expand. When the 

 longitudinal muscles contract the body 

 is shortened and must become corre- 

 spondingly thicker, which causes the 

 circular muscles to expand. The ac- 

 tivity of a muscle is always expressed 

 by contraction. When it expands it 

 is passive, the action being due to the 

 contraction of other muscles. The 

 worm is provided witri groups of 



FIG. 91. Plain muscle 

 fibres. n, Nucleus; p. 

 protoplasm; p', muscle 

 fibrils. 



FIG. 92. Three muscle cells of nereis in cross sec- 

 tion. The dots on the periphery of the cell are the 

 muscle fibrils in cross section. 



bristles (setae) on either side of each segment. These bristles 

 can be set forward or backward by means of small ifluscles 

 connected with them. The setae prevent slipping ofthe body 

 in the direction in which they are set. If now the setae are set 

 backward and the circular muscles contract, the anterior end of 

 the animal moves forward, the posterior end remaining fixed. 

 Then, when the longitudinal muscles contract, the posterior 

 end moves forward, and the anterior end remains fixed. A 



