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 
coelomic 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 coelomic cavities are found, which are directly comparable 
with those found in the vertebrate. 
