56 THE ORIGIN OF VERTEBRATES 
evolution of the animal kingdom, the cesophagus necessarily pierced 
the central nervous system at the cephalic end. At the same time, 
the very fact that the evolution was progressive necessitated the 
concentration and increase of the nervous masses in this very same 
cesophageal region. 
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 con- 
centrated 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 Palostracan 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 ner- 
vous material surrounds an ceso- 
phagus 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 cesophagus for the purpose 
of drawing blood through this 
exceedingly narrow tube. 
Fic. 26.— TRANSVERSE SECTION , : ; ; 
THROUGH THE BRAIN OF A YOUNG In Fig. 25 this increasing 
THELYPHONUS. antagonism between brain-power 
A, supra-cesophageal ganglia; B, infra- and alimentation, as we pass from 
cesophageal ganglia; 4/, esophagus. ch a form as Branchipus to the 
scorpion, is illustrated, and in Fig. 26 the relative sizes of the 
cesophagus and the brain-mass surrounding it is shown. The section 
shows that the food channel is surrounded by the white and grey 
