There is, however, one interesting question about these two larvæ which I should much like to solve. The ascidian has only _one_ eye inside its useless brain, while the tadpole and all other vertebrates have _two_ from the very first. Now which of us most nearly represents the old mud-loving vertebrate ancestor in this respect? Have two original organs coalesced in the young ascidian, or has one organ split up into a couple with the rest of the class? I think the latter is the true supposition, and for this reason: In our heads, and those of all vertebrates, there is a curious cross-connection between the eyes and the brain, so that the right optic nerve goes to the left side of the brain and the left optic nerve goes to the right side. In higher animals, this 'decussation,' as anatomists call it, affects all the sense-organs except those of smell; but in fishes it only affects the eyes. Now, as the young ascidian has retained the ancestral position of his almost useless eye so steadily, it is reasonable to suppose that he has retained its other peculiarities as well. May we not conclude, therefore, that the primitive vertebrate had only one brain-eye; but that afterwards, as this brain-eye grew outward to the surface, it split up into two, because of the elongated and flattened form of the head in swimming animals, while its two halves still kept up a memory of their former union in the cross-connection with the opposite halves of the brain? If this be so, then we might suppose that the other organs followed suit, so as to prevent confusion in the brain between the two sides of the body; while the nose, which stands in the centre of the face, was under no liability to such error, and therefore still keeps up its primitive direct arrangement.