10 



THE COCKROACH : 



usually transverse lines it remains thin, the chitinous layer, 

 though perfectly continuous, becoming extremely flexible, and 

 allowing a certain amount of deflection or retraction (fig. 1). 



Fig. 1. Diagram of Arthropod limb extended, retracted, and flexed. 

 Graber has given a similar figure (Insekten, fig. 8*). 



The joints of the trunk and limbs may thus resemble stiff tubes. 

 Muscles are attached to their inner surface, and are therefore 

 enclosed by the system of levers upon which they act (fig. 2fi). 

 In Vertebrate animals, on the contrary, which possess a true 

 internal skeleton, the muscles clothe the levers (bones) to which 



A B 



Fig. 2. Vertebrate and Arthropod joints. A, Vertebrate joint, the skeleton clothed 

 with muscles. B, Arthropod joint, the skeleton enclosing the muscles. 



they are attached (fig. 2 A.). The whole outer surface of an 

 Arthropod, including the eyes, auditory membrane (if there is 

 one), and surface-hairs, is chitinised. Chitin may also stiffen 



