256 THE ORIGIN OF VERTEBRATES 



interpreted. These two sets of muscle-segments correspond in the vertebrate 

 to the somatic and splanchnic segmentations. 



3. In the vertebrate the segmentation in this region is indicated by the 

 ccelomic or head-cavities, which are cavities formed in the mesoderm of the 

 embryo, the walls of which give origin to the striated muscles of the head. In 

 Limulus corresponding ccelomic cavities are found, which are directly comparable 

 with those found in the vertebrate. 



