518 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. XXVIII. 



backward and inward. The} T bend the frontal plates downward, and 

 thus assist in attaching the lunules (a, figs. 24 and 25). 



There are three pairs 

 of muscles which operate 

 in producing flexion be- 

 tween the cephalon and 

 the thorax along the 

 cross-bar of the H -shaped 

 groove already d e - 

 scribed. The shortest of 

 these three pairs are the 

 nearest to the median 

 line and nearly parallel 

 with it (b). The second 

 pair are very much larg- 

 er and extend from the 

 middle of the cross-bar 

 frontward and outward, 

 at an angle of 45 degrees, 

 to the very edge of the 

 carapace (c). 



The third pair are 

 again nearly parallel with 

 the mid line but much 

 farther away from it than 

 the first pair (d). 



The muscles for the 

 extension of the free 

 (fourth) thorax segment, 

 the genital segment, and 

 the abdomen arise side 

 by side in pairs near the 

 median line in the pos- 

 terior portion of the 

 thoracic area (e). The 

 outer pair are the short- 

 est and terminate near 

 the center of the fourth 

 segment, controlling the 

 movements of that seg- 

 ment. The pair next in- 

 side of these are the longest and extend through the genital segment 

 and into the abdomen. 



The third or inner pair terminate in the genital segment. 

 In connection with these we find shorter muscles farther back aris- 

 ing near the center of the genital segment and terminating in the 



Fig. 24.— Musculature op a male Caligus curtus. (After 

 Pickering and Dana.) a, flexors of frontal plates; b, 

 c, d, MUSCLES producing flexion between the cephalon 

 and thorax; e, extensors of free segment, genital seg- 

 ment and abdomen; f, extensors between the genital 



SEGMENT AND ABDOMEN; g, PRODUCE FLEXION BETWEEN THO- 

 RACIC AND LATERAL AREAS; h, i, MOTOR MUSCLES OF THE SEC- 

 OND AND THIRD LEGS; 1, MANDIBLE MOTORS. 



