Fig. 163.—Diagram of two possible methods of the Formation of a Notochord.
The position, then, of the notochord and its method of formation suggests that the mid-ventral surface of the arthropod ancestor of the vertebrate formed a deep groove between the bases of all the prosomatic, mesosomatic, and metasomatic appendages, which was subsequently converted into a tube extending along the whole of the body between mouth and anus, and finally, by the proliferation of its lining cells and their conversion into notochordal tissue, became the notochordal rod of the vertebrate.
As already frequently stated, Apus and Branchipus are the two living arthropods which most nearly resemble the extinct trilobites. The beautiful specimens of Triarthrus (Fig. [165]) found by Beecher give an idea of the under surface of the trilobite such as has never been obtained before, and demonstrate how closely the condition of things found in Apus (Fig. [164]) was similar to that occurring in the trilobites. In both cases the mid-ventral surface of the animal formed a deep groove which extended the whole length of the animal; on each side of this groove in Apus are closely set the gnatho-bases of the appendages, in such a manner that the groove can be easily converted into a canal by the movements of these bases—a canal which, owing to the great number of the appendages and their closeness to each other, can be completely and efficiently closed.
| Fig. 164.—Under-Surface of Apus. (After Bronn.) | Fig. 165.—Under-Surface of a Trilobite (Triarthrus). (From Beecher.) |
All those who have seen Apus in the living state assert that this canal so formed is actually used by the animal for feeding purposes. By the movements of the gnatho-bases food is passed up from the hind end of the animal along the whole length of this ventral canal to the mouth, where it is taken in and swallowed. In this way Apus has been seen to swallow its own eggs.
In the trilobites there is a similar deep channel formed by the mid-ventral surface, similar gnatho-bases, and closely set appendages, and the membrane of this ventral groove was extremely thin.
Here, then, in the very group of animals which were the progenitors of the presumed palæostracan ancestor of the vertebrate—a group which is characterized by its extensive prevalence and its enormous variety of form during the great trilobite era—the formation of a mid-ventral canal out of this deep ventral groove is seen to be not only easy to imagine, but most probable, provided that a necessity arose for such a conversion.
For what purpose might such a tube have been formed? I would suggest that it might have acted as an accessory food-channel, which was of sufficient value at the time to give some advantage in the struggle for existence to those members of the group who were thus able to supplement their intake of food, but at the same time was so inefficient that it was quickly superseded by the new alimentary canal, and thus losing its temporary function, became solid, and was utilized to form an axial supporting rod.
There is a very considerable amount of evidence in favour of the view that the notochord was originally a digestive tube; in fact, as far as I know, this conclusion is universally accepted. The evidence is based essentially upon its development and upon its structure. It is formed in the vertebrate from the same layer as the alimentary canal, i.e. the hypoblast, and in Amphioxus it commences as a groove in the dorsal wall of the future alimentary canal; this groove then closes to form the tube of the notochord, and separates from the alimentary canal. Embryologically, then, the notochord is looked upon as a tube formed directly from the alimentary canal.