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 Amnioccetes, 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 



