=General characteristics of Devonian fishes.= _The notochord is persistent._ The notochord is a continuous rod of cartilage, or gristle, which in the embryological growth of vertebrate animals supports the spinal nerve cord before the formation of the vertebræ. In most modern fishes and in all higher vertebrates the notochord is gradually removed as the bodies of the vertebræ are formed about it; but in the Devonian fishes it persists through maturity and the vertebræ remain incomplete. =The skeleton is cartilaginous.= This also is an embryological characteristic. In the Devonian fishes the vertebræ, as well as the other parts of the skeleton, have not ossified, or changed to bone, but remain in their primitive cartilaginous condition. [Illustration: Fig. 302. Vertebræ of Sturgeon in side view _A_; and vertical transverse section _B_, showing Notochord _ch_, and Neural Canal _m_] =The tail fin is vertebrated.= The backbone runs through the fin and is fringed above and below with its vertical rays. In some fishes with vertebrated tail fins the fin is symmetric (Fig. 300), and this seems to be the primitive type. In others the tail fin is unsymmetric: the backbone runs into the upper lobe, leaving the two lobes of unequal size. In most modern fishes (the _teleosts_) the tail fin is not vertebrated: the spinal column ends in a broad plate, to which the diverging fin rays are attached. But along with these embryonic characters, which were common to all Devonian fishes, there were other structures in certain groups which foreshadowed the higher structures of the land vertebrates which were yet to come: air sacs which were to develop into lungs, and cartilaginous axes in the side fins which were a prophecy of limbs. The vertebrates had already advanced far enough to prove the superiority of their type of structure to all others. Their internal skeleton afforded the best attachment for muscles and enabled them to become the largest and most powerful creatures of the time. The central nervous system, with the predominance given to the ganglia at the fore end of the nerve cord,--the brain,--already endowed them with greater energy than the invertebrates; and, still more important, these structures contained the possibility of development into the more highly organized land vertebrates which were to rule the earth.