The supra-infundibular portion of the brain in vertebrates corresponds clearly to the supra-œsophageal portion of the invertebrate brain in so far that in both cases here is the seat of the will. Voluntary action is as impossible to the arthropod deprived of its supra-œsophageal ganglia as to the vertebrate deprived of its cerebrum. It corresponds, also, in that from it arise the nerves of sight and smell and no other nerves; this is also the case with the supra-œsophageal ganglia, for from a portion of these ganglia arise the nerves to the eyes and the nerves to the first antennæ, of which the latter are olfactory in function. Thus, in the accompanying figure, taken from Bellonci, it is seen that the supra-œsophageal ganglia consist of a superior segment corresponding to the cerebrum, a middle segment from which arise the nerves to the lateral eyes and to the olfactory antennæ, corresponding to the basal ganglia of the brain and the optic lobes, and, according to Bellonci, of an inferior segment from which arise the nerves to the second pair of antennæ. This last segment is not supra-œsophageal in position, but is situated on the œsophageal commissures. It has been shown by Lankester and Brauer in Limulus and the scorpion to be in reality the first ganglion of the infra-œsophageal series, and not to belong to the supra-œsophageal group.
Fig. 27.—The Brain of Sphæroma serratum. (After Bellonci.)
Ant. I. and Ant. II., nerves to 1st and 2nd antennæ. f.br.r., terminal fibre layer of retina; Op. g. I., first optic ganglion; Op. g. II., second optic ganglion; O.n., optic nerve-fibres forming an optic chiasma.
Further, in Limulus, in the scorpion-group, and in all the extinct Eurypteridæ—in fact, in the Palæostraca generally—there are two median eyes in addition to the lateral eyes, which were innervated from these ganglia.
In Ammocœtes, then, if the supra-infundibular portion of the brain really corresponds to the supra-œsophageal of the palæostracan group, we ought to find, as indeed is the case, an optic apparatus consisting of two lateral eyes and two median eyes, innervated from the supra-infundibular brain-mass, and an olfactory apparatus built up on the same lines as in the scorpion-group, also innervated from this region. If, in addition, it be found that those two median eyes are degenerate eyes of the same type as the median eyes of Limulus and the scorpion-group, then the evidence is so strong as to amount to a proof of the correctness of the theory. This evidence is precisely what has been obtained in recent years, for the vertebrate did possess two median eyes in addition to the two lateral ones, and these two median eyes are degenerate eyes of the type found in the median eyes of arthropods and are not of the vertebrate type. Moreover, as ought also to be the case, they are most evident, and one of the pair is most nearly functional in the lowest perfect vertebrate, Ammocœtes.
Of all the discoveries made in recent years, the discovery that the pineal gland of the vertebrate brain was originally a pair of median eyes is by far the most important clue to the ancestry of the vertebrate, for not only do they correspond exactly in position with the median eyes of the invertebrates, but, being already degenerate and functionless in the lowest vertebrate, they must have been functional in a pre-vertebrate stage, thus giving the most direct clue possible to the nature of the pre-vertebrate stage. It is especially significant that in Limulus they are already partially degenerated. What, then, ought to be the structure and relation to the brain of the median and lateral eyes of the vertebrate if they originated from the corresponding organs of some one or other member of the palæostracan group?
This question will form the subject of the next chapter.
Summary.
The object of this book is to attempt to find out from what group of invertebrates the vertebrate arose; no attempt is made to speculate upon the causes of variation by means of which evolution takes place.