114 



GENERAL BIOLOGY 



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connective tissue, the periosteum. This is continu- 

 ous with a dense inelastic tendon that spreads over 

 and permeates the muscle bundles, forming the 

 perimysium. Muscle and bone are thus most 

 intimately bound together into a unit. In inverte- 

 brates with a chitin- 

 ous exoskeleton the 

 muscles are usually 

 attached to the hard 

 shell or to internal 

 plates that arise from 

 the shell. In forms 

 like the worms, in 

 which the whole body 

 contracts strongly, the 

 muscles are to be 

 found in sheets, usu- 

 ally running in differ- 

 ent directions, and 

 exerting a reciprocal 

 action on one another. 

 In the very lowest 

 and simplest forms 

 they occur as isolated fibers. In such cases they 

 represent the first step in the specialization of the 

 contractile function common to all protoplasm. 



Circulatory Systems. In most of the lower 

 animals, particularly those protected by a tough or a 

 hard outer covering, the tissues of the interior are 

 very soft and loosely held together. The organs are, 



FIG. 42. Diagrammatic cross-sec- 

 tion through the thorax of an insect, 

 showing the nature of the musculature 

 and the manner of the insertion of the 

 muscles to the exoskeleton : ht, heart ; 

 n, nerve cord ; wg, root of wing ; Ig, 

 root of leg ; ap, apodeme or chitinous 

 brace ; Im, longitudinal muscles of the 

 " back " ; m, dorso-ventral muscles oper- 

 ating wings and legs. (After Graber.) 



