CHAPTER XII 
THE REGION OF THE SPINAL CORD 
Difference between cranial and spinal regions.—Absence of lateral root.— 
Meristic variation —Segmentation of ceelom.—Segmental excretory organs. 
—Development of nephric organs; pronephric, mesonephric, metanephric. 
—Excretory organs of Amphioxus.—Solenocytes.— Excretory organs of 
Branchipus and of Peripatus, appendicular and somatic. —- Comparison 
of celom of Peripatus and of vertebrate——Pronephric organs compared to 
coxal glands.— Origin of vertebrate body-cavity (metaccele).—Segmental 
duct.—Summary of formation of excretory organs.--Origin of somatic 
trunk-musculature.—Atrial cavity of Amphioxus.—Pleural folds.— Ventral 
growth of pleural folds and somatic musculature.—Pleural folds of Cepha- 
laspidee and of Trilobita.—Significance of the ductless glands.— Alteration 
in structure of excretory organs which have lost their duct in vertebrates 
and in invertebrates.—Formation of lymphatic glands.—Segmental coxal 
glands of arthropods and of vertebrates.—Origin of adrenals, pituitary 
body, thymus, tonsils, thyroid, and other ductless glands.—Summary. 
THE consideration of the auditory nerve and the auditory apparatus 
terminates the comparison between the cranial nerves of the verte- 
brate and the prosomatic and mesosomatic nerves of the arthropod, 
and leaves us now free to pass on to the consideration of the verte- 
brate spinal nerves and the organs they supply. Before doing so, it 
is advisable to pass in review the conclusions already attained. 
Starting with the working hypothesis that the central nervous 
system of the vertebrate has arisen from the central nervous system 
of the arthropod, but has involved and enclosed the alimentary canal 
of the latter in the process, so that there has been no reversal of 
surfaces in the derivation of the one form from the other, we have been 
enabled to compare closely all the organs of the head-region in the 
two groups of animals, and in no single case have we been compelled 
to make any startling or improbable assumptions. The simple 
following out of this clue has led in every case in the most natural 
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