In our search, then, for a clue to the origin of the skeletal tissues of the vertebrate we see again that we are led directly to the palæostracan stock on the invertebrate side and to the Cyclostomata on that of the vertebrate; for in Limulus, the only living representative of the Palæostraca, and in Limulus alone, we find a skeleton marvellously similar to the earliest vertebrate skeleton—that found in Ammocœtes. Later on I shall give reasons for the belief that the earliest fishes so far found, the Cephalaspidæ, etc., were built up on the same plan as Ammocœtes, so that, in my opinion, in Limulus and in Ammocœtes we actually possess living examples allied to the ancient fauna of the Silurian times.
Summary.
The skeleton considered in this chapter is not the notochord, but that composed of cartilage. The tracing downwards of the vertebrate bony and cartilaginous skeleton to its earliest beginnings leads straight to the skeleton of the larval lamprey (Ammocœtes), in which vertebræ are not yet formed, but the cranial and branchial skeleton is well marked.
The embryological and phylogenetic histories are in complete unison to show that the cranial skeleton is older than the spinal, and this primitive branchial skeleton is also in harmony with the laws of evolution, in that its structure, even in the adult lamprey (Petromyzon), never gets beyond the stage characteristic of embryonic cartilage in the higher vertebrates.
The simplest and most primitive skeleton is that found in Ammocœtes and consists of two parts: (1) a prosomatic, (2) a mesosomatic skeleton.
The prosomatic skeleton forms a non-segmented basi-cranial skeleton of the simplest kind—the trabeculæ and the parachordals with their attached auditory capsules, just as the embryology of the higher vertebrates teaches us must be the case. There in the free-living, still-existent Ammocœtes we find the manifest natural outcome of the embryological history in the shape of simple trabeculæ and parachordals, from which the whole complicated basi-cranial skeleton of the higher vertebrates arose.
The mesosomatic skeleton, which is formed before the prosomatic, consisted, in the first instance, of simple branchial bars segmentally arranged, which were connected together by a longitudinal subchordal bar, situated laterally on each side of the notochord. These simple branchial bars later on form the branchial basket-work, which forms an open-work cage within which the branchiæ are situated.
The cartilages which compose these two skeletons respectively are markedly different in chemical constitution, in that the first (hard cartilage) is mainly composed of chondro-gelatin, the second (soft cartilage) of chondro-mucoid material.
The same kind of difference is seen in the two kinds of connective tissue which are the forerunners of these two kinds of cartilage. Thus, the cranial walls in Ammocœtes are formed of white fibrous tissue, an essentially gelatin-containing tissue; at transformation these are invaded by chondro-blasts and the cartilaginous cranium, formed of hard cartilage, results. On the other hand, the forerunner of the branchial soft cartilage is a very striking and peculiar kind of connective tissue loaded with mucoid material, to which the name muco-cartilage has been given.
The enormous interest of this muco-cartilage consists in the fact that it forms very well-defined plates of tissue, entirely confined to the head-region, which are not found in any higher vertebrate, not even in the adult form Petromyzon, for every scrap of the tissue as such disappears at transformation.