xxvii MUSCULAR SYSTEM 327 



muscles (M), which fill up a great part of the interior of the 

 body. Neither on the deric epithelium nor elsewhere are 

 there any cilia, the absence of these structures being gene- 

 rally characteristic of Arthropods. 



The cuticle (Cu) is of great thickness, and except at the 

 joints between the various segments of the body and limbs, 

 is impregnated with lime-salts so as to form a hard, jointed 

 armour. It thus constitutes a skeleton which, unlike that 

 of the starfish (p. 310), is a cuticular exoskeleton, forming a 

 continuous investment over the whole body but discon- 

 tinuously calcified. It is shed and renewed periodically 

 once a year during adult life the process being known as 

 ecdysis. 



The muscular system shows a great advance in complexity 

 over that of Polygordius, and consists entirely of transversely 

 striated fibres. In the abdomen the muscles are of great 

 size, and are divisible into a smaller dorsal and a larger 

 ventral set. The dorsal muscles (Fig. 86, em; Fig. 84, 

 d. m) are paired longitudinal bands, divided into segments 

 called myomeres, and inserted by connective tissue into the 

 anterior border of each segment : anteriorly they are trace- 

 able into the thorax, where they arise from the side-walls of 

 that region. When these muscles contract they draw the 

 anterior edge of each tergum under the posterior edge of 

 its predecessor, and thus extend or straighten the abdomen. 



The ventral muscles (Fig. 86, f m) are extraordinarily 

 complex. Omitting details, there is on each side a wavy 

 longitudinal band of muscle (Fig. 84, c.m), nearly circular in 

 section, which sends off a slip (ex) to be inserted into each 

 segment above the hinge : the contraction of this muscle 

 must obviously tend to approximate the terga, and so aid 

 the dorsal muscles in extending the abdomen. Around this 

 central muscle is wrapped, in each segment, a band of 



