VI EXOSKELETON AND MUSCLES 355 



articular membrane, movement being therefore possible in 

 any plane. 



Sections show the body-wall to consist of an integument 

 composed of a layer of deric epithelium (epiderm) secreting 

 a thick cuticle, and a layer of connective 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 characteristic 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 exoskekfon, forming 

 a continuous investment over the whole body but discon- 

 tinuously 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 complexity 

 over that of the earthworm : and consists entirely of 

 transversely striated fibres (compare p. 112). In the abdo- 

 men the muscles are of great size, and are divisible into a 

 smaller dorsal and a larger ventral set. The dorsal muscles 

 (Figs. 83 and 86, eni) are paired longitudinal bands, divided 

 into segments called myomeres (p. 203), and inserted by 

 connective tissue into the anterior border of each segment : 

 anteriorly 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. 



A A 2 



