The Origin of Vertebrates

by Walter Holbrook Gaskell

Among the whole of the invertebrates at present living on the earth, is there any sign of an internal cartilaginous skeleton that will give a direct clue to the origin of the primitive vertebrate skeleton? The answer to this question is most significant: only one animal among all those at present known possesses a cartilaginous skeleton, which is directly comparable with that of Ammocœtes, and here the comparison is very close—only one animal among the thousands of living invertebrate forms, and that animal is the only representative still surviving of the palæostracan group, which was the dominant race when the vertebrate first made its appearance. The Limulus, or king-crab, possesses a segmented branchial internal cartilaginous skeleton (Fig. [53], A), made up of the same kind of cartilage as the branchial skeleton of Ammocœtes, confined to the mesosomatic or branchial region, just as in Ammocœtes, forming, as in Ammocœtes, cartilaginous bars supporting the branchiæ, and these bars are situated externally to the branchiæ, as in Ammocœtes. In addition this animal possesses a basi-cranial internal semi-cartilaginous unsegmented plate known as the entosternite or plastron situated, with respect to the œsophagus, similarly to the position of the trabeculæ with respect to the infundibulum in Ammocœtes. Moreover, the cartilaginous cells in this tissue differ from those in the branchial region, in precisely the same manner as the hard cartilage differs from the soft in Ammocœtes.

This plastron, it is true, is found in other animals, all of which are members of the scorpion tribe, except in one instance, and this, strikingly enough, is the crustacean Apus—a strange primitive form, which is acknowledged to be the nearest representative of the Trilobita still living on the earth. None of these forms, however, possess any sign of an internal cartilaginous branchial skeleton, such as is possessed by Limulus. Scorpions, Apus, Limulus, are all surviving types of the stage of organization which had been reached in the animal world when the vertebrate first appeared.

The Mesosomatic or Respiratory Skeleton of Limulus, composed of Soft Cartilage.

Searching through the literature of the histology of the cartilaginous tissues in invertebrate animals, to see whether any cartilage had been described similar to that seen in the branchial cartilages of Ammocœtes, and whether such cartilage, if found, arose in a fibrous tissue resembling muco-cartilage, I was speedily rewarded by finding, in Ray Lankester's article on the tropho-skeletal tissues of Limulus, a picture of the cartilage of Limulus, which would have passed muster for a drawing of the branchial cartilage of Ammocœtes. This clue I followed out in the manner described in my former paper in the Journal of Anatomy and Physiology, and mapped out the topography of this remarkable tissue.

Limulus, like other water-dwelling arthropods, breathes by means of gills attached to its appendages. These gill-bearing appendages are confined to the mesosomatic region, as is seen in Fig. [59]; and these appendages are very different to the ordinary locomotor appendages, which are confined to the prosomatic region. Each appendage, as is seen in Fig. [58], consists mainly of a broad, basal part, which carries the gill-book on its under surface; the distal parts of the appendage have dwindled to mere rudiments and still exist, not for locomotor purposes, but because they carry on each segment organs of special importance to the animal (see Chapter XI.). As is seen in Fig. [58], the basal parts of each pair of appendages form a broad, flattened paddle, by means of which the animal is able to swim in a clumsy fashion. Very striking and suggestive is the difference between these gill-bearing mesosomatic appendages and the non-gill-bearing locomotor appendages of the prosoma.

Fig. 58.—Transverse Section through the Mesosoma of Limulus, to show the Anterior (A) and the Posterior (B) Surfaces of a Mesosomatic or Branchial Appendage.

In each figure the branchial cartilaginous bar, Br.C., has been exposed by dissection on one side. Ent., entapophysis; Ent.l., entapophysial ligament cut across; Br.C., branchial cartilaginous bar, which springs from the entapophysis; H., heart; P., pericardium; Al., alimentary canal; N., nerve cord; L.V.S., longitudinal venous sinus; Dv., dorso-ventral somatic muscle; Vp., veno-pericardial muscle.

At the base of each of these appendages, where it is attached to the body of the animal, the external chitinous surface is characterized by a peculiar stumpy, rod-like marking, and upon removing the chitinous covering, this surface-appearance is seen to correspond to a well-marked rod of cartilage (Br.C.), which extends from the body of the animal well into each appendage. This bar of cartilage arises on each side from the corresponding entapophysis (Ent.), which is the name given to a chitinous spur which projects a short distance (Fig. [58], B) into the animal from the dorsal side, for the purpose of giving attachment to various segmental muscles. These entapophyses are formed by an invagination of the chitinous surface on the dorsal side and are confined to the mesosomatic region, so that the mesosomatic carapace indicates, by the number of entapophyses, the number of segments in that region, in contradistinction to the prosomatic carapace, which gives no indication on its surface of the number of its components.

Each entapophysis is hollow and its walls are composed of chitin; but from the apex of each spur there stretches from spur to spur a band of tissue, called by Lankester the entapophysial ligament (Ent.l.) (Fig. [58]), and in this tissue cartilage is formed. Isolated cartilaginous cells, or rather groups of cells, are found here and there, but a concentration of such groups always takes place at each entapophysis, forming here a solid mass of cartilage, from which the massive cartilaginous bar of each branchial appendage arises.