Seeing that the steady progress of the development of the central nervous system is the most important factor in the evolution of animals, it follows that of all organs of the body, the central nervous system must be most easily comparable with that of the supposed ancestor. I will, therefore, start by comparing the brain of Ammocoetes with that of arthropods, especially of Limulus and of the scorpion-group. {62}The supra-infundibular portion of the brain in vertebrates corresponds clearly to the supra-oesophageal 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-oesophageal 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-oesophageal 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-oesophageal 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-oesophageal in position, but is situated on the oesophageal commissures. It has been shown by Lankester and Brauer in Limulus and the scorpion to be in reality the first ganglion of the infra-oesophageal series, and not to belong to the supra-oesophageal group. [Illustration: 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 {63}Eurypteridæ--in fact, in the Palæostraca generally--there are two median eyes in addition to the lateral eyes, which were innervated from these ganglia.