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This first rudimentary structure of the double nose is the same in all the Gnathostomes; it has no connection with the primitive mouth. But even in a section of the fishes a connection of this kind begins to make its appearance, a furrow in the surface of the skin running from each side of the nasal pit to the nearest corner of the mouth.
From the primitive fishes or Selachii, the earliest Gnathostomes, was developed the legion of the Ganoids. On the other hand, we have a great variety of specimens of this group in the fossil state, from the Upper Silurian onward. Some of these fossil Ganoids approach closely to the Selachii; others are nearer to the Dipneusts; others again represent a transition to the Teleostei.
We have to consider the fishes carefully as the class which, on the evidence of palaeontology, comparative anatomy, and ontogeny, may be regarded with absolute certainty as the stem-class of all the higher Vertebrates or Gnathostomes. Naturally, none of the actual fishes can be considered the direct ancestor of the higher Vertebrates.
In all the Gnathostomes the Wolffian duct is converted into the spermaduct, and the Mullerian duct into the oviduct. Only one of them is retained in each sex; the other either disappears altogether, or only leaves relics in the shape of rudimentary organs.
But it is certain that all the Vertebrates or Gnathostomes, from the fishes to man, descend from a common, extinct, fish-like ancestor. If we had this ancient stem-form before us, we would undoubtedly class it as a true fish. Fortunately the comparative anatomy and classification of the fishes are now so far advanced that we can get a very clear idea of these interesting and instructive features.
The labyrinth is divided into two vesicles in all the Gnathostomes. On the thin wall of this delicate labyrinth the acoustic nerve, which comes from the after-brain, spreads out in most elaborate fashion. The former seems to have more to do with the quality, the latter with the quantity, of the acoustic sensations.
But the EXTERNAL, superficial branchial skeleton that supports the gill-crate in the Cyclostoma is replaced in the Gnathostomes by an INTERNAL branchial skeleton. It consists of a number of successive cartilaginous arches, which lie in the wall of the gullet between the gill-clefts, and run round the gullet from both sides.
The distinction between lymph, chyle, and blood-vessels which is found in all the Craniotes may be regarded as an outcome of division of labour between various sections of our originally simple vascular system. In the Gnathostomes the spleen makes its first appearance, an organ rich in blood, the chief function of which is the extensive formation of new colourless and red cells.
The following characteristics of the Gnathostomes are anatomic features of this kind: The internal gill-arch apparatus with the jaw arches; the pair of nostrils; the floating bladder or lungs; and the two pairs of limbs. It is inherited in rudimentary form by all of them, from the earliest fishes to man.
In order to understand properly the genealogical tree of our race within the vertebrate stem, it is important to bear in mind the characteristics that separate the whole of the Gnathostomes from the Cyclostomes and Craniota. In these respects the fishes agree entirely with all the other Gnathostomes up to man, and it is on this that we base our claim of relationship to the fishes.
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