The Early Vertebrates and the Fishes 



are known only from their few living members, a circumstance 

 which is of course easily accounted for by the absence of any hard 

 parts capable of being preserved as fossils. From this point 

 onwards there is a skeleton, and we are consequently enabled to 

 draw valuable information from fossils. Partly in consequence 



Fig. 68. — Diagrammatic cross-section of lamprey larva. 



;;, Nerve cord ; cJi^ notochord ; ar^ artery ; ?', vem ; g^ repro- 

 ductive body ; d^ alimentary canal. 



of this, the Evolution chain from this point onwards is much more 

 complete than the portion that we have dealt with thus far. 



We have already observed that the true fishes, to which we 

 must now direct our attention, differ from the round-mouths in 

 several important characters. They possess two pairs of ex- 

 tremities, the pectoral and pelvic fins ; they have a pair of nostrils ; 

 there is also a well-developed skull, which includes a series of 

 cartilaginous or bony arches situated in the wall of the gut and 



89 



