Walter Holbrook Gaskell

“ I venture, then, to suggest that in the Osteostraci the median hard plate or glabellum protected a brain which was enclosed in a membranous cranium, very probably not yet complete in the dorsal region—certainly not complete if the median pineal eyes so universally found in these ancient fishes were functional—a cranium derived from the basal trabeculæ, in precisely the same manner as we see it already in its commencement in Phrynus and other scorpions. ”

“ The position appears to me to be this: the dermal cranial skeleton of vertebrates, whether it takes the form of a bony skull or of the dorsal plates of a cephalaspid or a pteraspid is, in all cases, not cuticular, i.e. is not an external formation of the epidermal cells, but is formed in tissue of the nature of connective tissue underlying the epidermis. On the contrary, the hard part of the head-carapace of the palæostracan is an external formation of the epidermal cells. ”

“ If, on the contrary, the spine is a terminal tail-spine, then, as far as the fossilized remains indicate, the animal consisted of a dorsal shield, a ventral shield, and a tail-spine, to which must be added two apparently lateral pieces and a few scales. If the animal did not possess a flexible body with a tail-fin, but terminated in a rigid spike after the fashion of a Limulus-like animal, then it must have moved by means of appendages. ”

“ The epidermal cells rest on a thick layer of most striking appearance (Fig. 141) , for it resembles, in an extraordinary degree, when examined superficially, a layer of chitin; it is called the laminated layer, and is characterized by the extreme regularity of the laminæ. This appearance is due, as the observations of Miss Alcock show, to alternate layers of connective tissue fibres arranged at right angles to each other, each fibre running a straight course and possessing its own nucleus. ”

“ At an age when the branchial cartilages are seen to be formed, when no fat-cells are present, a distinctive tissue (Fig. 133, B) is plainly visible in the velum and at the base of the tentacles, in the very position where in the more advanced Ammocœtes muco-cartilage exists. Taking, then, this tissue as our guide, the specimens show that the space between the skin and the visceral muscles in which the cartilaginous basket-work lies is filled with a similar material. ”

“ The resemblance between the structure of the head-shield of Thyestes and the muco-cartilage of Ammocœtes, is most valuable, for muco-cartilage is unique, occurs in no other vertebrate, and every trace of it vanishes at transformation ”

“ With the completion of this cranium and its conversion into cartilage, and subsequently into bone, an efficient protection was afforded to the most vital part of the animal, and thus the hard head-shield of the Palæostraca and of the earliest fishes was gradually supplanted by the protecting bony cranium of the higher vertebrates.
Step by step it is easy to follow in the mind's eye the evolution of the vertebrate cranium, and because it was evolved direct from the plastron, the impossibility of resolving it into segments is at once manifest ”

“ In Ammocœtes the layer which represents the covering of the Palæostracan has already almost disappeared. At transformation the layers representing the stage arrived at by the Heterostracan and the Osteostracan disappear; but the stage representing the higher vertebrates, far from disappearing, by the formation of cartilage reaches a higher stage and prepares the way for the ultimate stage of all—the formation of the bony cranium. ”

“ Although the fibres in each layer are packed close together, they are sufficiently apart to form with the fibres of the alternate layers a meshwork rather than a homogeneous structure, and thus the surface view of this layer shows a regular network of very fine spaces through which nerve-fibres and fluid pass. ”

“ Keep the animal the right side uppermost, and the evidence of the rocks confirms the transition from the Palæostracan to the Cyclostome; reverse the surfaces, and the attempt to derive the vertebrate from the palæostracan becomes so confused and hopelessly muddled as to throw discredit on any theory of the origin of vertebrates from arthropods. For my own part, I fully expect that appendages will be found not only in the Cephalaspidæ but also in the Pteraspidæ, and I hope Patten will continue his researches with increasing success. ”

“ There are always two ways of looking at a question, and it seems to me possible and more probable to turn the matter round and to argue that the original condition of the surface-covering was that of large plates, as in Pteraspis. By the subsequent splitting up of such plates, Drepanaspis was formed, and later on, by further splitting, the elasmobranch, Thelodus being a stage on the way to the formation of an elasmobranch, and not a backward stage from the elasmobranch towards Pteraspis. ”

“ From such a comparison it follows that the segments, found in the antero-lateral region of the head-shield, were not segments of the cranium, but of parts beyond the region of the cranium, and from their position must have been segments supplied by the trigeminal nerve, and not by the vagus group; segments, therefore, which did not indicate gills and gill-slits, but muscles, innervated by the trigeminal nerve; muscles which, as indicated by the corresponding markings on the carapace of Phrynus, Mygale, etc., were the tergo-coxal muscles of the prosomatic appendages. ”

“ That the animal crawled about in the mud by means of free appendages is by no means an impossible view, seeing how difficult it is to find the remains of appendages in the fossils of this far-back time, even when we are sure that they existed. ”

“ Among living animals, as I have shown, the Limulus is the sole survivor of the palæostracan type, and Ammocœtes alone gives a clue to the nature of the cephalaspid, i.e. the osteostracan fish. Older than the latter is the heterostracan, Pteraspis, and Cyathaspis. ”

“ Of the two head-shields, ventral and dorsal, the latter is best known and characterizes the group. It consists of a dorsal plate, with characteristic horns, which in Thyestes verrucosus (Fig. 128) , as described by Rohon, is composed of two parts, a frontal part and an occipital part (occ.) , the occipital part being composed of segments, and possessing a median ridge—the crista occipitalis. ”

“ The covering somatic muscles in the branchial region form a dorsal and ventral group, of which the latter is formed in the embryo much later than the former, the line of separation between the two groups being the lateral groove, with its row of branchial openings. This groove ends at the first branchial opening, but the ventral and dorsal somatic muscles continue further headwards. It is instructive to see that, although the lateral groove terminates, the separation between the two groups of muscles is still marked out by a ridge of muco-cartilage, represented in Fig. ”

“ If, then, this tissue of Pteraspis is not to be looked upon as chitin, how can we imagine its formation? It is certainly not bone, for there are no bone-corpuscles; it is a very regular laminated structure resembling in appearance chitin rather than anything else.
As in all cases of difficulty, turn to Ammocœtes and let us see what clue there is to be found there. The skin of Ammocœtes is peculiar among vertebrates in many respects. It consists of a number of epidermal cells, as in Fig. ”

“ For this purpose Shipley kindly allowed me to examine his series of sections through the embryo at various ages. These specimens are very instructive, especially those stained by osmic acid, which preserves the natural thickness of this space better than other staining methods. ”

“ The dorsal head-shield of muco-cartilage covers over the upper lip, sends a median prolongation over the median pineal eyes and a lateral prolongation on each side as far as the auditory capsules, giving the shape of the head-shield of muco-cartilage, as in Fig. ”

“ These are divided into three orders: (1) the Heterostraci, including one family, the Pteraspidæ, to which Pteraspis and Cyathaspis belong; (2) the Osteostraci, divisible into two families, the Cephalaspidæ and Tremataspidæ, which include Cephalaspis, Eukeraspis, Auchenaspis or Thyestes, and Tremataspis; and (3) the Antiarcha, with one family, the Astrolepidæ, including Astrolepis, Pterichthys, and Bothriolepis. Of these, the first two orders belong to the Upper Silurian, while the third is Devonian.
The Dorsal Head-Shield of the Osteostraci. ”