466 



CHORD ATA 



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 outlined has a 

 great influence on the rest of the organism. As already pointed out, the 

 external appearance of vertebrates has been altered by it, since the skin 

 is no longer a supporting structure and has consequently lost its segmen- 

 tation. More immediate is its influence upon the arrangement of the 



FIG. 523. Fie. 524. 



FIG. 523. Horizontal section through the anterior trunk region of a young Rhodeus 

 amarus at the level of the ventral arches, c, notochord; h, skin; li, myosepta; m, 

 longitudinal muscles; r, rib end of the cartilaginous ventral arch; v, osseous centrum. 



FIG. 524. Horizontal section of an embryo of Triton (from O. Hertwig). ch, 

 notochord; uh, myoccele; z*s,primary muscle segment (myotome). 



musculature. The development of an internal skeleton renders it neces- 

 sary that the point of resistance of the muscles be transferred from the 

 skin to the interior. A dermal musculature occurs only as an incon- 

 spicuous remnant; it is replaced by a body musculature. This latter 

 consists primarily of a system of longitudinal muscle fibres on either side 

 of the vertebral column (fig. 523), which are divided by connective-tissue 

 partitions, the myosepta, into successive segments, the myotomes. Thus 

 when the connective tissue of a fish is dissolved by cooking, the muscles 

 fall into disk-like parts. The myosepta extend from skin to axial skeleton. 

 Since they run obliquely backwards from the skeleton to the skin, they 

 serve to render the skeleton a point of resistance for the action of the 

 muscles. 



A segmented trunk musculature occurs in the Myxinoids (and in 

 Amphioscus), in which the axial skeleton consists only of notochord and is 

 consequently un jointed. The segmentation of the muscles is therefore older 

 than that of the skeleton and, as we can further say, is the cause of it. 



