Amongst the vertebrata the degree of intellect and an approach in structure to man clearly come into play.
The paddle-fins of the dipneusta changed into five-toed legs, which were afterwards transmitted to the higher vertebrata up to man.
Out of certain coelomati, the most ancient skull-less vertebrata were directly developed.
But all vertebrata have the great nerve trunk at the back, contained in the spine and with a bulb on the front or upper end constituting the brain.
We have taken our illustrations from only the two departments of vertebrata and articulata, because these are the most familiar to the reader, and also have been most carefully studied.
The mammifers first disappeared, then the mighty birds, then the reptiles of the secondary period, presently the fish, the crustacea, the mollusks, and finally the vertebrata.
