The Origin of Vertebrates

by Walter Holbrook Gaskell

The branchial cartilages and sub-chordal ligaments are coloured blue, the branchiæ red. gl., glandular substance surrounding the central nervous system and passing into the auditory capsule with the auditory nerve (VIII.).

In Fig. [59] I have shown the prosoma and mesosoma of Limulus, and indicated the nerves to the appendages together with the mesosomatic cartilaginous skeleton.

In Fig. [60] I have drawn a corresponding picture of the prosomatic and mesosomatic region of Ammocœtes with the corresponding nerves and cartilages. In this figure the animal is supposed to be slit open along the ventral mid-line and the central nervous system exposed.

The Prosomatic Skeleton of Limulus, composed of Hard Cartilage.

The rest of the primitive vertebrate skeleton arose in the prosomatic region, and formed a support for the base of the brain. This skeleton was composed of hard cartilage, and arose in white fibrous tissue containing gelatin rather than mucin.

Is there, then, any peculiar tissue of a cartilaginous nature in Limulus and its allies, situated in the prosomatic region, which is entirely separate from the branchial cartilaginous skeleton, which acts as a supporting internal framework, and contains a gelatinous rather than a mucoid substratum?

It is a striking fact, common to the whole of the group of animals to which our inquiries, deduced from the consideration of the structure of Ammocœtes, have, in every case, led us in our search for the vertebrate ancestor, that they do possess a remarkable internal semi-cartilaginous skeleton in the prosomatic region, called the entosternite or plastron, which gives support to a large number of the muscles of that region; which is entirely independent of the branchial skeleton, and differs markedly in its chemical reactions from that cartilage, in that it contains a gelatinous rather than a mucoid substratum.

In Limulus it is a large, tough, median plate, fibrous in character, in which are situated rows and nests of cartilage-cells. The same structure is seen in the plastron of Hypoctonus, of Thelyphonus, and to a certainty in all the members of the scorpion group. Very different is the behaviour of this tissue to staining from that of the branchial region. No part of the plastron stains purple with thionin; it hardly stains at all, or gives only a very slight blue colour. In its chemical composition there is a marked preponderance of gelatin with only a slight amount of a mucin-body. In some cases, as in Hypoctonus (Fig. [57], B) and Mygale, the capsules of the cartilage-cells stain a deep yellow with hæmatoxylin and picric acid, while the fibres between the cell-nests stain a blue-brown colour, partly from the hæmatoxylin, partly from the picric acid.

All the evidence points to the plastron as resembling the basi-cranial skeleton of Ammocœtes in its composition and in the origin of its cells in a white fibrous tissue. What, then, is its topographical position? It is in all cases a median structure lying between the cephalic stomach and the infra-œsophageal portion of the central nervous system, and in all cases it possesses two anterior horns which pass around the œsophagus and the nerve-masses which immediately enclose the œsophagus (Fig. [61], A). These lateral horns, then, which lie laterally and slightly ventral to the central nervous system, and are called by Ray Lankester and Benham the sub-neural portion of the entosternite, are very nearly in exactly the position of the racquet-shaped head of the trabeculæ in Ammocœtes. It is easy to see that, with a more extensive growth of the nervous material dorsally, such lateral horns might be caused to take up a still more ventral position. Now, these two lateral horns of the plastron of Limulus are continued along its whole length so as to form two thickened lateral ridges, which are conspicuous on the flat surface of the rest of this median plate. In other cases, as in the Thelyphonidæ, the plastron consists mainly of these two lateral ridges or trabeculæ, as they might be called, and Schimkéwitsch, who more than any one else has made a comparative study of the entosternite, describes it as composed in these animals of two lateral trabeculæ crossed by three transverse trabeculæ. I myself can confirm his description, and give in Fig. [61], B, the appearance of the entosternite of Thelyphonus or of Hypoctonus. The supra-œsophageal ganglia and part of the infra-œsophageal ganglia fill up the space Ph.; stretching over the rest of the infra-œsophageal mass is a transverse trabecula, which is very thin; then comes a space in which is seen the rest of the infra-œsophageal mass, and then the posterior part of the plastron, ventrally to which lies the commencement of the ventral nerve-cord.