512 VERTEBRATE LIFE AND ORGANIZATION 



Aliliough ilic appenclicuhn skeleton oi the dogfish is quite different 

 from that ol terrestrial vertebrates, there is a close resemblance between 

 the appendicular skeleton of crossopterygian fishes and tetrapods (Fig. 

 25.7). The humerus oi our arm, or the femur of our leg, represents the 

 single proximal bone of the crossopterygian fin; the radius and ulna, 

 or tibia and fibula, the next two l)ones. The carpals or tarsals, meta- 

 tarsals or metacarpals, and phalanges of the hand or foot are homol- 

 ogous with the more peripheral elements of the crossopterygian fin. We 

 tetrapods have a single bone in the proximal part of the appendage 

 followed by two bones in the second part because this pattern was es- 

 tablished by our piscine ancestors. 



The girdles of tetrapods are necessarily stronger than those of fish. 

 The pectoral girdle is bound onto the body by muscles, but the pelvic 

 girdle extends dorsally and is firmly attached to the vertebral column. 

 A pubis, ischium and ilium are present on each side of our pelvic girdle, 

 though all have fused together in the adult. Our pectoral girdle includes 

 a scapula, a coracoid process, which is a distinct bone in most lower 

 tetrapods, and a clavicle. The clavicle is the only remnant of a series of 

 dermal bones that are primitively associated with the girdle. All other 

 girdle bones are cartilage replacement bones. 



216. Muscles 



The movement of the vertebrate body and its parts, and the posture 

 of the vertebrate body, depend upon the contraction of muscles. The 

 nature of muscle contraction and the source of the energy required have 

 been considered earlier. At this time we will be concerned with certain 

 aspects of the evolution of the muscular system, and the relation of these 

 to changes in methods of locomotion. 



Histologically, muscles may be classified as smooth, cardiac and 

 skeletal. In tracing their evolution it is more convenient to divide them 

 into somatic muscles associated with the body wall and appendages, 

 and visceral muscles associated with the pharynx and other parts of the 

 gut tube. This grouping parallels the major subdivisions of the skeletal 

 system. Somatic muscles are striated and under voluntary control. Most 

 of the visceral muscles are smooth and involuntary; however, the visceral 

 muscles associated with the visceral arches, called branchial muscles, are 

 striated and under voluntary control. 



Most of the somatic musculature of fishes consists of segmented myo- 

 meres (Fig. 25.8). This is an effective arrangement for bringing about 

 the lateral undulations of the trunk and tail that are responsible for 

 locomotion. The muscles of the paired fins are very simple, and consist 

 of little more than a single dorsal extensor that pulls the fin up and 

 caudally, and a ventral flexor that pulls the fin down and anteriorly. 



The transition from water to land entailed major changes in the 

 somatic muscles. The appendages became increasingly important in loco- 

 motion, and movements of the trunk and tail less important. The 

 primitive single fin extensor and flexor became divided into many com- 

 ponents, and these became larger and more powerful. Despite the 



