It is this evidence of primitive non-vertebrate tissues, which occur in the larval but not in the adult form, which makes Ammocoetes 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. {147}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 Ammocoetes is extraordinarily great. Here, in Limulus, just as in Ammocoetes, an internal cartilaginous skeleton is found, composed of two distinct parts: (1) prosomatic, (2) mesosomatic. As in Ammocoetes, 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 Ammocoetes, and, as in the latter case, arises in a markedly mucoid connective tissue. The former, as in Ammocoetes, 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.