IV. VERTEBRATA. 531 



must be transferred from the skin, where it is found in annelids, 

 molluscs, and arthropods, to the interior. A dermal musculature 

 occurs only as an inconspicuous remnant in vertebrates; it is re- 

 placed by a body musculature. This latter consists primarily of a, 

 longitudinal system of muscle fibres on either side of the vertebral 

 column (fig. 566), which are divided by connective-tissue partitions,, 

 the myosepta or myocommata, into successive segments, the myo- 

 tomes. Thus when the connective tissue of a fish is dissolved by 

 cooking the muscles fall into disk-like parts. The myosepta ex- 

 tend from skin to axial skeleton. Since they run obliquely back- 

 wards 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 Amphioxus), in which the axial skeleton consists only of noto- 

 chord 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. The action of the muscles prevents, 

 the formation of a cartilaginous or bony vertebral continuum such 

 as the notochord and skeletaginous layer are. It produces at in- 

 tervals joints or flexible parts separating the cartilaginous or bony 

 column into vertebrae. Naturally these flexible portions cannot, 

 coincide with the boundaries of the muscles, but must lie between 

 them; in other words, muscle segments and skeletal segments 

 myotomes and sclerotomes must alternate. Segmentation is lack- 

 ing in the cranium, since the myotomes here have no locomotor 

 significance, are reduced, and only small remnants of them persist. 



In the mammals only a little of this segrnental arrangement of 

 muscles is recognizable, a result of the 

 development of the appendages; and the 

 more these gain in importance as the 

 locomotor structures, the more the mus- 

 cles are modified and grouped for the ser- 

 vice of the limbs, so that only the inter- 

 costals and a part of the muscular system 

 to the sides of the vertebral column show 

 clearly the primitive metamerism. Yet 

 in all vertebrate embryos the muscles ap- 

 pear at first strictly segmental, in the 

 form of the primitive somites (fig. 567), 

 formerly called protovertebrse. tome). 



Another important point in the musculature lies in the fact 

 that it is dorsal in origin and therefore in fishes is largely dorsal 



