530 



CHORDATA. 



must be divided into sections, jointed to each other. By this 

 there are developed two joints of importance in both fore and hind 

 limbs; the elbow (knee) joint between humerus (femur) on the 

 one hand and radius and ulna (tibia and fibula) on the other; and 

 the wrist joint (ankle) between the bones of the fore arm (shank) 

 and the carpals (tarsals). Less important are the joints of the 

 fingers and toes. 



If the limbs of terrestrial vertebrates be compared with this 

 primary form, variations are seen in two directions. Rarely are 

 there more bones than in the schema; then there occur remnants 

 of a sixth or even a seventh row or finger. More frequently there 

 is a reduction in the number of parts, either by fusion or by abso- 

 lute loss. Fusion accounts for the fact that with complete pen- 

 tadactyly the number of carpalia is usually less than ten, as would 

 be expected from the schema. Degeneration and loss explain the 

 existence of animals with four, three, two, or even one digit, and 

 one can say with certainty that the missing parts are in most cases 

 lost, though a fusion of digits is not unknown. Paleontology, for 

 example, teaches that the one-toed horse has descended, by gradual 

 reduction, from five-toed ancestors. 



The completeness and character of the skeleton thus sketched 



FIG. 566. Horizontal section through the anterior trunk region of a young Rhodeus 

 amarus at the level of the ventral arches, c, notochord ; /i, skin ; //, interrnuscu- 

 lar ligament; w, longitudinal muscles ; r, rib end of the cartilaginous ventral 

 arch ; v, osseous centrum. 



in outline has a great influence on the rest of the organism. It 

 has already been pointed out that the external appearance of ver- 

 tebrates has been influenced by it, since the skin is no longer, as in 

 arthropods, a supporting structure and has consequently lost its 

 segmentation. More immediate is its influence upon the arrange- 

 ment of the musculature. The development of an internal skele- 

 ton renders it necessary that the point of resistance of the muscles. 



