ZOOLOG Y. 
53 
stages of existence in the Vertebrata, behind the mouth, 
thickenings ; they are known as the visceral arches. (Fig. 
78, c.) The spaces between these thickenings finally dis- 
appear, 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 Verte- 
brata 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 root- 
like foot. The gulf between the Vertebrates and Inverte- 
brates is now bridged over by this discovery of the identical 
development of the Amphioxus and Ascidia. The Am- 
phioxus 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 
