Fig. 16.—Tadpole of Frog and of Ascidian. Surface view.
Fig. 17.—Tadpole of Frog and of Ascidian. Diagrams representing the chief internal organs.
Now let us look at the internal organs (Fig. 17). There are four structures, which are all four possessed at some time of their lives by all those animals which we call the Vertebrata, the great branch of the pedigree to which fishes, reptiles, birds, beasts, and men belong. And the combination of these marks or structural peculiarities is an overwhelming piece of evidence in favour of the supposition that the creatures which possess this combination are derived from one common ancestor. Just as one would conclude that a man whom one might meet, say on Salisbury Plain, must belong to the New Zealand race, if it were found not only that he had the colour, and the hair, and the shape of head of a New Zealander, but also that he was tattooed like a New Zealander, carried the weapons of a New Zealander, and, over and above in addition to these proofs, that he talked the Maori language and none other; so here, in the case of the vertebrate race, there are certain qualities and possessions, the accumulation of which cannot be conceived of as occurring in any animal but one belonging to that race. These four great structural features are—first, the primitive backbone or notochord; second, the throat perforated by gill-slits; third, the tubular nerve-centre or spinal cord and brain placed along the back; and lastly, and perhaps most distinctive and clinching as an evidence of affinity, the myelonic or cerebral eye.
Now let us convince ourselves that these four features exist not only in the frog’s tadpole, as they do in all fishes, reptiles, birds, and beasts, but that they also exist in the Ascidian tadpole, and, it may be added, co-exist in no other animals at all.
The corresponding parts are named in Figs. 16 and 17, in such a way as to render their agreement tolerably clear, whilst in Fig. 18 a more detailed representation of the head of an Ascidian tadpole is given.
Fig. 18.—Ascidian Tadpole with a part only of the tail C. N, nervous system with the enlarged brain in front and the narrow spinal cord behind n; N´, is placed in the cavity of the brain: O, the single cerebral eye lying in the brain; a, similarly placed auditory organ; K, pharynx; d, intestine; o, rudiment of the mouth; ch, notochord or primitive backbone. (From Gegenbaur’s “Elements of Comparative Anatomy.”)