vn EXOSKELETON AND MUSCLES 369 



tion being formed merely by an annular articular mem- 

 brane, movement being therefore possible in any plane. 



Sections show the body- wall to consist of an integu- 

 ment composed of a layer of deric epithelium or epiderm 

 (p. 329) secreting a thick cuticle, and a layer of con- 

 nective-tissue forming the derm, beneath which is a 

 very thick layer of large and complicated muscles which 

 fill up a great part of the interior of the body. Neither 

 on the epiderm nor elsewhere are there any cilia, the 

 absence of these structures being generally character- 

 istic of Arthropods. 



The cuticle 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 cuticular 

 exoskeleton, forming a continuous investment over the 

 whole body but discontinuously calcified. It is shed 

 entire and renewed periodically once a year during 

 adult life the process being known as ecdysis, growth 

 taking place during the period between ecdysis and 

 renewal while the animal is soft. 



The muscular system shows a great advance in com- 

 plexity over that of the earthworm, and consists 

 entirely of transversely striated fibres (compare p. 112). 

 In the abdomen the muscles are of great size, and are 

 divisible into a smaller dorsal and a larger ventral 

 set. The dorsal muscles (Figs. 90 and 93, em) are 

 paired longitudinal bands, divided into segments called 

 myomeres (p. 203), and inserted by connective-tissue 

 into the anterior .border of each segment : in front they 

 are traceable 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 (compare p. 63). 



The ventral muscles (Figs. 90 and 93, fm) are 



PR ACT. ZOOL. B B 



