In the preceding chapter the primitive cartilaginous vertebrate skeleton, as found in Ammocoetes, was shown to correspond in a marvellous manner to the cartilaginous skeleton of Limulus. In a later chapter I will deal with the formation of the cranium from the prosomatic skeleton; in this chapter it is the mesosomatic skeleton which is of interest, and the consideration of the necessary consequences which logically follow upon the supposition that the branchial cartilaginous bars of Limulus are homologous with the branchial basket-work of Ammocoetes.
Seeing that in both cases the cartilaginous bars of Limulus and Ammocoetes are confined to the branchial region, their homology of necessity implies an homology of the two branchial regions, and leads directly to the conclusion that the branchiæ of the vertebrate were derived from the branchiæ of the arthropod, a conclusion which, according to the generally accepted view of the origin of the respiratory region in the vertebrate, is extremely difficult to accept; for the branchiæ of Limulus and of the Arthropoda in general are part of the mesosomatic appendages, while the branchiæ of vertebrates are derived from the anterior part of the alimentary canal. This conclusion, therefore, implies that the vertebrate has utilized in the formation of the anterior portion of its new alimentary canal the branchial appendages of the palæostracan ancestor.
{ 150}[Illustration: FIG. 62.--_Eurypterus._
The segments and appendages on the right are numbered in correspondence with the cranial system of lateral nerve-roots as found in vertebrates. _M._, metastoma. The surface ornamentation is represented on the first segment posterior to the branchial segments. The opercular appendage is marked out by dots.]
Let us consider dispassionately whether such a suggestion is _a priori_ so impossible as it at first appears. One of the principles of evolution is that any change which is supposed to have taken place in the process of formation of one animal or group of animals from a lower group must be in harmony with changes which are known to have occurred in that lower group. On the assumption, therefore, that the vertebrate branchiæ represent the branchial portion of the arthropod mesosomatic appendages which have sunk in and so become internal, we ought to find that in members of this very group such inclusion of branchial appendages has taken place. This, indeed, is exactly what we do find, for in all the scorpion tribe, which is acknowledged to be closely related to Limulus, there are no external mesosomatic appendages, but in all cases these appendages have sunk into the body, have disappeared as such, and retained only the vital part of them--the branchiæ. In this way the so-called lung-books of the scorpion are formed, which are in all respects homologous with the branchiæ or gill-books of Limulus. Now, as already mentioned, the lords of creation in the palæostracan times were the sea-scorpions, which, as is seen in Fig. 62, resembled the land-scorpions of the present day in the entire absence of any external appendages on the segments of the mesosomatic region. As they lived in the sea, they must have breathed with gills, and those branchial appendages must have been internal, just as in the land-scorpions of the present time. Indeed, markings have been found on the internal side of the segments 1-5, Fig. 62, which are supposed to indicate branchiæ, and these segments are therefore supposed to have borne the branchiæ. Up to the present time no indication of gill-slits has been found, and we cannot say with certainty how these animals breathed. Further, in the Upper Silurian of Lesmahago, Lanarkshire, a scorpion (_Palæophonus Hunteri_), closely resembling the modern scorpion, has been found, which, as Lankester states, was in all probability aquatic, and not terrestrial in its habits. How it { 151}breathed is unknown; it shows no signs of stigmata, such as exist in the scorpion of to-day.