ARTHROPODA. 91 



3. The muscular system is conveniently considered here, since 

 its arrangement is dependent upon the structure of the exoskeleton, 

 to the inside of which the muscles are attached. 



Locomotion consists in swimming, walking, or climbing. The 

 first results from alternate flexion (bending ventralwards), and 

 extension (straightening), of the abdomen. In accordance with 

 this, two great flexor muscles arise from the roof of the sternal 

 canal, and are inserted into the abdominal sterna, and two much 

 smaller extensor muscles have their origin and insertion in the side- 

 walls of the thorax and abdominal terga respectively. During 

 flexion the intertergal membranes are stretched and the inter- 

 sternal ones folded, vice versd during extension. The lateral peg- 

 and-socket joints only permit upward and downward movements, 

 while excessive extension is prevented by the overlapping terga. 

 Movements of the walking-legs and forceps effect the other two 

 kinds of locomotion, and the successive segments of these append- 

 ages are connected by hinge-joints, the axes of which all take 

 different directions, so that the limb can be bent in various planes 

 and execute complex movements. Some of the limb-muscles are 

 extrinsic, taking origin from the exoskeleton of the body ; others 

 are intrinsic, having both origin and insertion within the limb. 



The jaws are moved to and from the middle line by adductor 

 and abductor muscles. The mandibular adductors are easily seen 

 in dissection. The claws of the first three pairs of thoracic limbs 

 are opened and shut by abductor and adductor muscles which 

 work the terminal joint against the produced part of the last 

 joint but one. In many cases it is advantageous for a large 

 muscle to be attached to a small and definite area. This is 

 managed by means of a firm tendon, formed as a cuticular infolding 

 to which the muscle-fibres are connected. 



The muscles are made up of numerous slender muscle-fibres 

 resulting from the modification of cells. Each of these fibres 

 is invested by a delicate membranous sheath (sarcolemma), and, 

 owing to the regular alternation of dark and clear bands, is 

 transversely striated. Hence the term "striped" or "striated" 

 muscle. Longitudinal striations also occur, and along these the 

 fibre can be split, after death, into primitive fibrillce. Beneath 

 the sarcolemma a number of longitudinal rows of muscle-corpuscles 

 are arranged, each of which consists of a nucleus surrounded by 

 a small quantity of protoplasm. 



