CHAPTER Xir 



THE REG 10 X OF THE SPINAL CORD 



Difference between cranial and spinal regions. — Absence of lateral root. — 

 Meristic variation. — Segmentation of cceloni. — Segmental excretory organs. 

 — Development of nepliric organs ; pronephric, mesonephric, metanepliric. 

 —Excretory organs of Ampliioxus.— Solenocytes. -Excretory organs of 

 Branchipus and of Peripatus, appendicular and somatic. — Comparison 

 of coelom of Peripatus and of vertebrate. — Pronephric organs compared to 

 coxal glands.— Orig-in of vertebrate body-.cavity (metacoele). — Segmental 

 duct. — Summary of formation of excretory organs.— Origin of somatic 

 trunk-musculature. — Atrial cavity of Ampliioxus. — Pleural folds. — Ventral 

 growth of pleural folds and somatic musculature. — Pleural folds of Cepha- 

 laspidae and of Trilobita. — Significance of the ductless glands. — Alteration 

 in structure of excretory org-ans 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 



2 c 



