GNA THOS TO MA TA 



59 



segmental animals, and correlated with this structure is the seg- 

 mentation of their muscular system. In the lower forms, and 

 in the embryos of the higher, the segmental character of the 

 somatic muscles is most distinctly shown. Now the connective 

 tissues are found to surround the myotomes, forming not only 

 closed boxes in which these lie, but also a lining to the body-wall 

 outside and to the body-cavity within, and a sheath surrounding 

 the notochord and central nervous system. Thus a system of 

 transverse septa (myocomata), intersegmental in position, and of 



msv. pr. 



Diagram of the connective-tissue system in the trunk of a Craniate Vertebrate, showing the 

 relation borne by the axial skeleton to the transverse and longitudinal septa, a.e, wall of 

 abdominal coelom ; bd, basidorsal ; bv, basiventral; b.w, cut body-wall; d.r, dorsal rib; i, 

 intestine hanging in the coelom ; iv, interventral ; TO, transverse septum (myOcomma) ; ms, 

 mesentery; m.s.d, median dorsal septum; m.s.v, median ventral septum ; nes, neural tube ; 

 n.s, notochordal sheath ; p.r, ventral or pleural rib ; s.p, neural spine ; ts, horizontal septum. 

 Oblique view of left side, from which the septa have been partially removed. 



longitudinal, more or less tubular coverings, is formed. To these 

 may be added a longitudinal vertical median septum, a longitudinal 

 horizontal septum (in Gnathostomes), and others of minor import- 

 ance, completing the system of membranes in which the various 

 parts of the skeleton arise (Fig. 40). 



The true endoskeleton, in fact, is only the local strengthening 

 of certain regions of the connective tissue, chiefly for the sake of 

 affording a firmer hold and greater mechanical advantage to the 

 muscular system enclosed in it. Since the musculature is seg- 

 mented, we find that the skeletal system is also, as a rule, of a 

 segmental character. As the lateral paired limbs become differen- 

 tiated, and as the head becomes more and more * cephalised ' 



