It is this evidence of primitive non-vertebrate tissues, which occur in the larval but not in the adult form, which makes Ammocœtes so valuable for the investigation of the origin of vertebrates.
The evidence, then, is extraordinarily clear as to the beginnings of the vertebrate skeletal tissues.
In the invertebrate kingdom true cartilage occurs but scantily. There is a cartilaginous covering of the brain of cephalopods. It is never found in crabs, lobsters, bees, wasps, centipedes, butterflies, flies, or any of the great group of Arthropoda, except, to a slight extent, in some members of the scorpion group, and more fully in one single animal, the King-crab or Limulus: a fact significant of itself, but still more so when the nature of the cartilage and its position in the animal is taken into consideration, for the identity both in structure and position of this internal cartilaginous skeleton with that of Ammocœtes is extraordinarily great.
Here, in Limulus, just as in Ammocœtes, an internal cartilaginous skeleton is found, composed of two distinct parts: (1) prosomatic, (2) mesosomatic. As in Ammocœtes, the latter consists of simple branchial bars, segmentally arranged, which are connected together on each side by a longitudinal ligament containing cartilage—the entapophysial ligament. This cartilage is identical in structure and in chemical composition with the soft cartilage of Ammocœtes, and, as in the latter case, arises in a markedly mucoid connective tissue. The former, as in Ammocœtes, consists of a non-segmental skeleton, the plastron, composed of a white fibrous connective tissue matrix, an essentially gelatin-containing tissue, in which are found nests of cartilage cells of the hard cartilage variety.
This remarkable discovery of the branchial cartilaginous bars of Limulus, together with that of the internal prosomatic plastron, causes the original difficulty of deriving an animal such as the vertebrate from an animal resembling an arthropod to vanish into thin air, for it shows that in the past ages when the vertebrates first appeared on the earth, the dominant arthropod race at that time, the members of which resembled Limulus, had solved the question; for, in addition to their external chitinous covering, they had manufactured an internal cartilaginous skeleton. Not only so, but that skeleton had arrived, both in structure and position, exactly at the stage at which the vertebrate skeleton starts.
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.
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 Ammocœtes.—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.