vi.] GENERAL VIEW OF INTERNAL SKELETON. 225 



Sometimes, as in the Sharks, not only are there solid 

 splanchnapophyses, but there are at the same time external 



FIG. 195. DIAGRAM OF THE CONDITION OF THE SKELETON IN THE BRANCHIAL 

 REGION OF A LAMPREY (transverse vertical section). 



w, neural canal; b, alimentary canal, only bounded by b ', which is a membra- 

 nous representative of splanchnapophysial parts ; v, the ascending vessels ; 

 /, the paraxial system (external cartilaginous skeleton of the gills), or branchial 



paraxial cartilages (like those of the Lamprey) coexisting 

 with the splanchnapophysial hard parts of the branchial 

 region. 



FIG. 196. DIAGRAM OF THE CONDITION OF THE SKELETON IN THE BRANCHIAL 

 REGION OF SOME SHARKS (transverse vertical section). 



, neural canal ; b, alimentary canal, surrounded by solid visceral hypaxial 

 parts (splanchnapophyses) ; v, the aortic vessels, extending up outside the 

 branchial arches and inside the paraxial system (/), here represented by 

 Certain external branchial cartilages. 



The general summary of the endo-skeleton here given 

 refers, of course, to the skeleton in its most developed con- 

 dition, and expresses the greatest complexities exhibited to 

 us amongst the whole series of vertebrate animals. But, 

 as the most perfect skeleton is at one time an embryonic 

 structure devoid of both bone and cartilage, so in the 

 lowest of Vertebrates the skeleton is (as we have seen) in 

 a soft, membranous condition. In lieu of the complex dif- 

 ferentiation of higher forms, we find in Amphioxus only 

 sheets of membrane, or lamellae, placed more or less at right 

 angles to the very long axis of the body and proceeding out- 

 wards from the notochord to the skin. 



The whole series of such membranous body-girdles may 

 Q 



