8. The coxal glands of the protostracan ancestor existed on all the segments, and were, therefore, divisible into three groups, prosomatic, mesosomatic, and metasomatic; these three groups of coxal glands still exist in the vertebrate as ductless glands.
9. The prosomatic coxal glands form the pituitary body.
10. The mesosomatic coxal glands form the thymus, thyroid, parathyroids, tonsils, etc.
11. The metasomatic coxal glands form the adrenals.
12. The procœlom of the vertebrate is the procœlom of the protostracan ancestor, which splits into a dorsal part, the myocœle, and a ventral part, the nephrocœle. This latter part not only forms the pronephros and mesonephros, but also by a ventral extension gives origin to the walls of the vertebrate body-cavity or metacœle.
13. This ventral extension of the original nephrocœle at first excreted to the exterior, through abdominal pores, or through peritoneal funnels. When such paths to the exterior became closed, it also became a ductless gland, belonging to the lymphatic system.
CHAPTER XIII
THE NOTOCHORD AND ALIMENTARY CANAL
Relationship between notochord and gut.—Position of unsegmented tube of notochord.—Origin of notochord from a median groove.—Its function as an accessory digestive tube.—Formation of notochordal tissue in invertebrates from closed portions of the digestive tube.—Digestive power of the skin of Ammocœtes.—Formation of new gut in Ammocœtes at transformation.—Innervation of the vertebrate gut.—The three outflows of efferent nerves belonging to the organic system.—The original close contiguity of the respiratory chamber to the cloaca.—The elongation of the gut.—Conclusion.
In the previous chapters all the important organs of the arthropod have been found in the vertebrate in their appropriate place, of similar structure, and innervated from corresponding parts of the central nervous system. Such comparison is possible only as long as the ventral and dorsal surfaces of the vertebrate correspond with the respective surfaces of the arthropod, and no reversal is assumed. This method of comparative anatomy is the surest and most certain guide to the relationship between two animals, and when the facts obtained by the anatomical method are so strikingly confirmatory of the palæontological evidence, the combined evidence becomes so strong as to amount almost to a certainty that vertebrates did arise from arthropods in the manner mapped out in previous chapters, and not from a hypothetical group of animals, such as is postulated in the theory of their origin from forms like Balanoglossus.