206 THE ORIGIN OF VERTEBRATES 
with in the comparison of the ciliated lining of the tube in the 
central nervous system of vertebrates with the chitinous lining 
of the intestine in the arthropod. Such a difference does not seem 
to me either unlikely or unreasonable, seeing that cilia are found 
instead of chitin in the intestine of the primitive arthropod Peri- 
patus. Also the worm-like ancestors of the arthropods almost 
certainly possessed a ciliated intestine. Finally, the researches of 
Hardy and McDougall on the intestine of Daphnia point directly to 
the presence of a ciliated rather than a chitinous epithelial lining of 
the intestine in this animal—all evidence pointing to the probability 
that in the ancient arthropod forms, derived as they were from the 
annelids, the intestine was originally ciliated and not chitinous. It 
is from such forms that I suppose vertebrates to have sprung, and 
not from forms like the living king-crabs, scorpions, Apus, Bran- 
chipus, etc. I only use them as illustrations, because they are the 
only living representatives of the great archaic group, from which 
the Crustacea, Arachnida, and Vertebrata all took origin. 
The second difference is more important, and is at first sight 
fatal to any comparison between the two organs. How is it possible 
to compare the uterus of the scorpion, which opens on the surface by 
an external genital opening, with the thyroid of Ammoccetes, which 
opens by an internal opening into the respiratory chamber? However 
close may be the histological resemblance of structure in the two 
cases, surely such a difference is too great to be accounted for. 
It is, however, to be remembered that the operculum of Scorpio 
covers only the terminal genital apparatus, and does not, therefore, 
resemble the operculum of the presumed ancestor of Ammoccetes, 
which, as already argued, must have resembled the operculum of 
Thelyphonus with its conjoint branchial and genital apparatus, 
rather than that of Scorpio. Before, therefore, making too sure of 
the insuperable character of this difficulty, we must examine the 
uterus of the Pedipalpi, and see the nature of its opening. 
The nature of the terminal genital organs in Thelyphonus has 
been described to some extent by Blanchard, and more recently by 
Tarnani. The ducts of the generative organs terminate, according to 
the latter observer, in the large uterus, which is found both in the 
male and female; he describes the walls of the uterus in the female 
as formed of elongated glandular epithelium, with a_ strongly- 
developed porous, chitinized intima. In the male, he says that the 
