The transition from the gill-bearing to the lung-bearing vertebrates is most interesting, for it has been proved that the lungs are formed by the modification of the swim-bladder of fishes; and in a group of fishes, the Dipnoi, or lung-fishes, of which three representatives still exist on the earth, the mode of transition from the fish to the amphibian is plainly visible, for they possess both lungs and gills, and yet are not amphibians, but true fishes. But for the fortunate existence of Ceratodus in Australia, Lepidosiren in South America, and Protopterus in Africa, it would have been impossible from the fossil remains to have asserted that any fish had ever existed which possessed at the same moment of time the two kinds of respiratory organs, although from our knowledge of the development of the amphibian we might have felt sure that such a transitional stage must have existed. Unfortunately, there is at present no likelihood of any corresponding transitional stage being discovered {149}living on the earth in which both the dorsal arthropod alimentary canal and the ventral vertebrate one should simultaneously exist in a functional condition; still it seems to me that even if Ceratodus, Lepidosiren, and Protopterus had ceased to exist on the earth, yet the facts of comparative anatomy, together with our conception of evolution as portrayed in the theory of natural selection, would have forced us to conclude rightly that the amphibian stage in the evolution of the vertebrate phylum was preceded by fishes which possessed simultaneously lungs and gills. In the preceding chapter the primitive cartilaginous vertebrate skeleton, as found in Ammocoetes, was shown to correspond in a marvellous manner to the cartilaginous skeleton of Limulus. In a later chapter I will deal with the formation of the cranium from the prosomatic skeleton; in this chapter it is the mesosomatic skeleton which is of interest, and the consideration of the necessary consequences which logically follow upon the supposition that the branchial cartilaginous bars of Limulus are homologous with the branchial basket-work of Ammocoetes. INTERNAL BRANCHIAL APPENDAGES.