GAMMARUS. 353 



tive system), the muscles may be divided into (1) those 

 moving the segments of the body, and (2) those moving 

 the appendages. 



(1) The muscles of the segments may be divided into 

 flexors and extensors. Of the former there are in each 

 segment two sets, oblique muscles and muscles running 

 parallel to the ventral wall of the body. In the thorax 

 and first three segments of the abdomen, the oblique 

 muscles, of which there are a pair in each segment, pass 

 from the ventral surface of one segment to half-way up 

 the body- wall of the next one behind. In the last three 

 segments of the abdomen, the oblique muscles are 

 inserted in the opposite way, running from half-way up 

 one segment to the ventral surface of the one behind. 

 This difference is, probably, due to the fact that when 

 the abdomen is flexed, the last three segments are bent 

 so completely under the body as to describe a curve 

 opposite in direction to that formed by the rest of the 

 body. The oblique muscles of the segments produce a 

 telescoping movement. In the lower part of the body, 

 pairs of muscles (PL IV., fig. 1, long, m. 1 ) run parallel to 

 the ventral surface, on either side of the middle line, 

 joining the middle of one segment to that of the next. 

 The points of insertion of each pair approximate very 

 closely to those of the next, so that they appear to form 

 a continuous double chain of muscle along the ventral 

 surface of the body. In the last three abdominal seg- 

 ments, which are flexed simultaneously, these muscles 

 are not segmented and attached to the middle of each 

 segment, but form a continuous double band from the 

 third abdominal segment to the sixth. 



The extensors are much stronger than the flexors, 

 forming broad bands of longitudinal muscle (PI. III., 

 fig. 1, long, m.), joining the front edge of the dorsal part 



