The Origin of Vertebrates

by Walter Holbrook Gaskell

The earliest fishes, then, ought to have possessed segmented pleural folds, which were moved by somatic muscles, and enveloped the body after the fashion of Ammocœtes and Amphioxus, and I cannot help thinking that Cephalaspis shows, in this respect also, its relation to Ammocœtes. It is well known that some of the fossil representatives of the Cephalaspids show exceedingly clearly that these animals possessed a very well-segmented body, and it is equally recognized that this skeleton is a calcareous, not a bony skeleton, and does not represent vertebræ, etc. It is generally called an aponeurotic skeleton, meaning thereby that what is preserved represents not dermal plates alone, or a vertebrate skeleton, but the calcified septa or aponeuroses between a number of muscle-segments or myomeres, precisely of the same kind as the septa between the myomeres in Ammocœtes. The termination of such septa on the surface would give rise to the appearance of dermal plates or scutes, or the septa may even have been attached to something of the nature of dermal plates. The same kind of picture would be represented if these connective tissue dissepiments of Ammocœtes were calcified, and the animal then fossilized. In agreement with this interpretation of the spinal skeleton of Cephalaspis, it may be noted that again and again, in parts of these dissepiments, I have found in old specimens of Ammocœtes nodules of cartilage formed, and at transformation it is in this very tissue that the spinal cartilages are formed.

Fig. 161.—A, Facsimile of Woodward's Drawing of a Specimen of Cephalaspis Murchisoni, as seen from the side. The Cephalic Shield is on the Right and Caudal to it the Pleural Fringes are well shown; B, Another Specimen of Cephalaspis Murchisoni taken from the same block of Stone, showing the Dermoseptal Skeleton and in one place the Pleural Fringes, bc.

Now, the specimens of Cephalaspis all show, as seen in Fig. [161], that the skeletal septa cover the body regularly, and then along one line are bent away from the body to form, as it were, a fringe, or rather a free pleuron, which has been easily pushed at an angle to the body-skeleton in the process of fossilization. Patten thinks that this fringed appearance is evidence of a number of segmental appendages which were jointed to the corresponding body-segments, and in the best specimen at the South Kensington Natural History Museum he thinks such joints are clearly visible. He concludes, therefore, that the cephalaspids were arthropods, and not vertebrates. I have also carefully examined this specimen, and do not consider that what is seen resembles the joint of an arthropod appendage; the appearance is rather such as would be produced if the line of attachment of Patten's appendages to the body were the place where the pleural body folds became free from the body, and so with any pressure a bending or fracture of the calcified plates would take place along this line. There is, undoubtedly, an appearance of finish at the termination of these skeletal fringes, as though they terminated in a definitely shaped spear-like point, just as is seen in the trilobite pleuræ. This, again, to my mind, is rather evidence of pleural fringes than of true appendages.

Fig. 162.—A, Arrangement of Septa in Ammocœtes (NC., position of notochord); B, Arrangement of Septa in Amphioxus.

As already argued, I look upon Ammocœtes as the only living fish at all resembling the cephalaspids; it is therefore instructive to compare the arrangement of this spinal dermo-septal skeleton of Cephalaspis with that of the septa between the myomeres in the trunk-region of Ammocœtes and Amphioxus. Such a skeleton in Ammocœtes would be represented by a series of plates overlapping each other, arranged as in Fig. [162], A, and in Amphioxus as in Fig. [162], B. I have lettered the corresponding parts of the two structures by similar letters, a, b, c. Ammocœtes differs in configuration from Amphioxus in that it possesses an extra dorsal (a, d) and an extra ventral bend. Ammocœtes is a much rounder animal than Amphioxus, and both the dorsal and ventral bends are on the extreme ventral and dorsal surfaces—surfaces which can hardly be said to exist in Amphioxus. The part, then, of such an aponeurotic skeleton in Ammocœtes which I imagine corresponds to b, c in Amphioxus, and therefore would represent the pleural fold, is the part ventral to the bend at b. In both the animals this bend corresponds to the position of the notochord NC.

The skeleton of Cephalaspis compares more directly with that of Ammocœtes than that of Amphioxus, for there is the same extra dorsal bend (Fig. [161], a, d) as in Ammocœtes; the lateral part of the skeleton again gives an angle a, b, c; the part from b to c would therefore represent the pleural fold. I picture to myself the sequence of events somewhat as follows:—

First, a protostracan ancestor, which, like Peripatus, possessed appendages on every segment into which cœlomic diverticula passed, forming a system of coxal glands; such glands, being derived from the segmental organs of the Chætopoda, discharged originally to the exterior by separate openings on each segment. It is, however, possible, and I think probable, that a fusion of these separate ducts had already taken place in the protostracan stage, so that there was only one external opening for the whole of these metasomatic coxal glands, just as there is only one external opening for the corresponding prosomatic coxal glands of Limulus. Then, by the ventral growth of pleural body-folds, such appendages became enclosed and useless, and the coxal glands of the post-branchial segments, with their segmental or pronephric duct, were all that remained as evidence of such appendages. This dwindling of the metasomatic appendages was accompanied by the getting-rid of free appendages generally, in the manner already set forth, with the result that a smooth fish-like body-surface was formed; then the necessity of increasing mobility brought about elongation by the addition of segments between those last formed and the cloacal region. Each of such new-formed segments was appendageless, so that its segmental organ was not a coxal gland, but entirely somatic in position, and formed, therefore, a mesonephric tubule, not a pronephric one. Such glands could no longer excrete to the exterior, owing to the enclosing shell of the pleural folds; but the pronephric duct was there, already formed, and so these nephric tubules opened into that, instead of, as in the case of the branchial slits, forcing their way through the pleural walls when the atrium became closed.

The Meaning of the Ductless Glands.