stages of existence in the Vertebrata, behind the mouth, thickenings; they are known as the visceral arches. (Fig. 178, c.) The spaces between these thickenings finally disappear, so that the interior of the mouth communicates with the exterior. Such a condition is retained in fishes, where we see the water entering the mouth and passing out through the gills. The visceral arches are never seen among the Invertebrates. Such structures as those just mentioned were often quoted as separating the Vertebrates entirely from the Invertebrates; and these differences, as well as others, were so great that they were considered as offering an insuperable objection to the view that the Vertebrata had been developed from the Invertebrata. Recently it has been shown, however, that the Ascidia, one of the Tunicate sac-worms, develops in the same manner as the Amphioxus, the simplest fish known. The young Ascidian (Fig. 38, a) resembles a tadpole, and swims freely about by means of its tail. In this state it has as much of a backbone as the Amphioxus. After it matures it becomes stationary (Fig. 38), remaining attached to objects by means of a rootlike foot. The gulf between the Vertebrates and Invertebrates is now bridged over by this discovery of the identical development of the Amphioxus and Ascidia. The Amphioxus is the only living representative of a group probably long since extinct. This group, allied to the sac-worms in its structure, has in one direction retrograded, the Ascidians, in another progressed, the Amphioxus.
The structure of the skull offers one of the most striking proofs for the common origin of the Vertebrata. If we compare in this respect a fish, turtle, bird, mouse, elephant, and man, we shall find that, notwithstanding the great difference in appearance of these animals, their skulls are fundamentally composed of the same bones arranged in the same manner.
Remembering the different uses of the arm of man and
