What the precise steps are by which chitin-formation gives place to chondrin-formation are not yet fully known, but Schmiedeberg has shown that a substance, glycosamine, is derivable from both these skeletal tissues, and he concludes his observations in the following words: "Thus, by means of glycosamine, the bridge is formed which connects together the chitin of the lower animals with the cartilage of the more highly organized creations." The evidence of the origin of the cartilaginous skeleton of the vertebrate points directly to the origin of the vertebrate from the Palæostraca, and is of so strong a character that, taken alone, it may almost be considered as proof of such origin. {148}CHAPTER IV _THE EVIDENCE OF THE RESPIRATORY APPARATUS_ Branchiæ considered as internal branchial appendages.--Innervation of branchial segments.--Cranial region older than spinal.--Three-root system of cranial nerves, dorsal, lateral, ventral.--Explanation of van Wijhe's segments.--Lateral mixed root is appendage-nerve of invertebrate.--The branchial chamber of Ammocoetes.--The branchial unit, not a pouch but an appendage.--The origin of the branchial musculature.--The branchial circulation.--The branchial heart of the vertebrate.--Not homologous with the systemic heart of the arthropod.--Its formation from two longitudinal venous sinuses.--Summary. The respiratory apparatus in all the terrestrial vertebrates is of the same kind--one single pair of lungs. These lungs originate as a diverticulum of the alimentary canal. On the other hand, the aquatic vertebrates breathe by means of a series of branchiæ, or gills, which are arranged segmentally, being supported by the segmental branchial cartilaginous bars, as already mentioned in the last chapter.