The Origin of Vertebrates

by Walter Holbrook Gaskell

Fig. 61.—A, Entosternite of Limulus; B, Entosternite of Thelyphonus.

Ph., position of pharynx.

In these forms, in which the central nervous system is more concentrated towards the cephalic end than in Limulus, the whole of the concentrated brain-mass is separated from the gut only by this thin transverse band of tissue. Judging, then, from the entosternite of Thelyphonus, it is not difficult to suppose that a continuation of the same growth of the brain-region of the central nervous system would cause the entosternite to be separated into two lateral trabeculæ, which would then take up the ventro-lateral position of the two trabeculæ of Ammocœtes.

On the other hand, it might be that two lateral trabeculæ, similar to those of Thelyphonus and situated on each side of the central nervous system, were the original form from which, by the addition of transverse fibres running between the gut and nervous system, the entosternite of Thelyphonus and of the scorpions, etc., was formed. From an extensive consideration of the entosternite in different animals, Schimkéwitsch has come to the conclusion that this latter explanation is the true one. He points out that the lateral trabeculæ can be distinguished from the transverse by their structure, being much more cellular and less fibrous, and the cell-cavities more rounded, or, as I should express it, the two lateral trabeculæ are more cartilaginous, while the transverse are more fibrous. Schimkéwitsch, from observations of structure and from embryological investigations, comes to the conclusion that the entosternite was originally composed of two parts—

1. A transverse muscle corresponding to the adductor muscle of the shell of certain crustaceans, such as Nebalia.

2. A pair of longitudinal mesodermic tendons, which may have been formed originally out of a number of segmentally arranged mesodermic tendons, and are crossed by the fibrils of the transverse muscular bundles.

These paired tendons of the entosternite he considers to correspond to the intermuscular tendons, situated lengthways, which are found in the ventral longitudinal muscles of most arthropods.

It is clear from these observations of Schimkéwitsch, that the essential part of the entosternite consists of two lateral trabeculæ, which were originally tendinous in nature and have become of the nature of cartilaginous tissue by the increase of cellular elements in the matrix of the tissue: these two trabeculæ function as supports for the attachment of muscles, which are specially attached at certain places. At these places transverse fibres belonging to some of the muscular attachments cross between the two longitudinal trabeculæ, and so form the transverse trabeculæ.

I entirely agree with Schimkéwitsch that the nests of cartilage-cells are much more extensive in, and indeed nearly entirely confined to, these two lateral trabeculæ in the entosternite of Hypoctonus. Ray Lankester describes in the entosternite of Mygale peculiar cell-nests strongly resembling those of Hypoctonus, and he also states that they are confined to the lateral portions of the entosternite.

From this evidence it is easy to see that that portion of the basi-cranial skeleton known as the trabeculæ may have originated from the formation of cartilage in the plastron or entosternite of a palæostracan animal. Such an hypothesis immediately suggests valuable clues as to the origin of the cranium and of the rest of the basi-cranial skeleton—the parachordals and the auditory capsules. The former would naturally be a dorsal extension of the more membranous portion of the plastron, in which, equally naturally, cartilaginous tissue would subsequently develop; and the reason why it is impossible to reduce the cranium into a series of segments would be self-evident, for even though, as Schimkéwitsch thinks, the plastron may have been originally segmented, it has long lost all sign of segmentation. The latter would be derived from a second entosternite of the same nature as the plastron, but especially connected with the auditory apparatus of the invertebrate ancestor. The following out of these two clues will be the subject of a future chapter.