21 
port of the body, we have a segment of an articulate creature, with 
only an histiological difference, which must by no means be allowed 
to conceal from us the true nature of a part (Geoffroy St. Hilaire, 
Sur la Vertébre, ut antea, p. 92). 
3. If to this view it should be objected, that the including in the 
one case what is excluded in the other dispels all semblance of homo- 
logy, it must be answered— 
a. That notwithstanding this difficulty, the general homology of 
the vertebrate and articulate skeletons as wholes has long been ad- 
mitted, though this more particular one of their parts has not been. 
B. That the heemal arch of the Vertebrata, whose normal office it 
is to enclose the main blood-vessels of the body, and which office it 
exclusively performs in many cases, is yet in others so developed as 
to enclose a mass of viscera, viz. in the thorax. 
y- In the Testudina we have an example of those vertebral ele- 
ments which are usually internal, becoming external, and including 
not only all the viscera, but having the whole muscular system at- 
tached internally, as in the Articulata, and even the limbs arising 
from the inside instead of the outside of the thorax. 
4. It presents no difficulty that the segments of the Articulata 
have no superior or inferior arches like vertebree, because both the 
spinal cord and circulatory organs which those arches are respectively 
designed to protect are included within the body (St. Hilaire ut 
antea, p. 102). 
5. To the order of development of a vertebra in the lateral pro- 
cesses for locomotion being produced subsequently to the body, we 
have an analogous case in that the Myriapoda are at birth and for 
some time afterwards apodal, and subsequently acquire their nume- 
rous legs (Newport on Myriapoda, Phil. Trans. 1841). This is also 
the case with some other articulate animals. 
Section VI. 
The brain of the Vertebrata is a modification of a series of four 
ganglia homologous with those of the spinal cord. 
1. In the Amphiovus that part of the cord which must be regarded 
as the homologue of the brain, because it gives off five pair of ce- 
phalic nerves, is only distinguished from the other part of the cord 
by its pointed anterior extremity, its posterior part being entirely 
like the other ganglia; even its greatest vertical diameter is not 
greater (De Quatrefages on Amphioxus, Annales des Scien. Nat., 
third series, vol. iv.). 
2." We have already noticed that the two large cephalic ganglia of 
the Centipede are the result of the coalescence of a series of four 
ganglia, as they appear in the fcetal condition, each of these nervous 
centres supplying nerves to the senses. Closely corresponding with 
this arrangement is that displayed by many of the fish, as e.g. the 
Eel, where the brain is only a series of four closely arranged ganglia. 
And this same original scheme seems to me traceable throughout all 
the Vertebrata to man himself. There are, however, as the great 
