Ur? 
ZOOLOG Y. LE 
constitutes Paleontology. This scince has shown that 
forms so different as the Horse and Rhinoceros are linked 
together by the fossil forms of Coryphodon, Paleotherium, 
etc.; that the Pig and Hippopotamus represent another 
group, connected by Anoplotherium and Dichotrum, etc. ; 
that the Fishes and Batrachia form one great division, the 
Reptiles and Birds another,—forms linking indissolubly 
together these divisions of the Vertebrates having been 
discovered in different parts of the world. The structure, 
the development, and fossil remains all harmonize in prov- 
ing without a doubt that they are only the modified posterity 
of a class now extinct, which the Amphioxus nearly repre- 
sents. That the Amphioxus came from Worms in their 
structure allied to Ascidians, is highly probable. But, given 
the Amphioxus, that the genealogy of the Vertebrata is 
represented by a tree, like that provisionally offered in Tree 
V., seems to us to follow, without doubt, from the facts of 
Anatomy, Geology, and Embryology. 
FISHES. 
The Amphioxus, or Lancelet, is a little animal about two 
inches long, found generally buried in the sand on the 
coasts of different seas. It does not possess head, brain, 
eyes, or limbs, and yet there exists a backbone in a rudi- 
mentary condition (notochord), and marrow. Its gills are 
not like those of fishes, but its branchial apparatus is that 
of an Ascidian (Fig. 40), confirming the view of the origin 
of the Amphioxus from the Ascidian Worms, suggested by 
their identical development. What is the Amphioxus? It 
seems to be an intermediate animal, a link connecting the 
Ascidian with the Fishes. The part of the body contain- 
ing the mouth is usually regarded as the head, and is mem- 
branous in structure, which condition is found in fishes at 
certain periods of their existence. We may designate the 
