Progress on these lines must result in a crisis, owing to the inevitable squeezing out of the food-channel by the increasing nerve-mass; and, indeed, the fact that such a crisis had in all probability arisen at the time when vertebrates first appeared is apparent when we examine the conditions at the present time.
Those invertebrates whose central nervous system is most concentrated at the cephalic end belong to the arachnid group, among which are included the various living scorpion-like animals, such as Thelyphonus, Androctonus, etc.
As already mentioned, the giants of the Palæostracan age were Pterygotus, Slimonia, etc., all animals of the scorpion-type--in fact, sea-scorpions. Now, all these animals, spiders and scorpions, without exception, are blood-suckers, and in all of them the concentrated cephalic mass of nervous material surrounds an oesophagus the calibre of which is so small that nothing but a fluid pabulum can be taken into the alimentary canal; and even for that purpose a special suctorial apparatus has in some species been formed on the gastric side of the oesophagus for the purpose of drawing blood through this exceedingly narrow tube.
In Fig. 25 this increasing antagonism between brain-power and alimentation, as we pass from such a form as Branchipus to the scorpion, is illustrated, and in Fig. 26 the relative sizes of the oesophagus and the brain-mass surrounding it is shown. The section shows that the food channel is surrounded by the white and grey matter of the brain as completely as the central canal of the spinal cord of the vertebrate is surrounded by the white and grey nervous material.
[Illustration: FIG. 26.--TRANSVERSE SECTION THROUGH THE BRAIN OF A YOUNG THELYPHONUS.
_A_, supra-oesophageal ganglia; _B_, infra-oesophageal ganglia; _Al_, oesophagus.]
{ 57}Truly, at the time when vertebrates first appeared, the direction and progress of variation in the Arthropoda was leading, owing to the manner in which the brain was pierced by the oesophagus, to a terrible dilemma--either the capacity for taking in food without sufficient intelligence to capture it, or intelligence sufficient to capture food and no power to consume it.
Something had to be done--some way had to be found out of this difficulty. The atrophy of the brain meant degeneration and the reduction to a lower stage of organization, as is seen in the Tunicata. The further development of the brain necessitated the establishment of a new method of alimentation and the closure of the old oesophagus, its vestiges still remaining as the infundibular canal of the vertebrate, meant the enormous upward stride of the formation of the vertebrate.