56 



THE ORIGIN OF VERTEBRATES 



evolution of the animal kingdom, the oesophagus 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 

 oesophageal 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 Palaeostracan age were 



Pterygotus, Slimonia, etc., all animals of the scorpion-type — in fact, 



A sea - scorpions. Now, all these 



,S "•'.. animals, spiders and scorpions, 



without exception, are blood - 

 suckers, and in all of them the 

 concentrated cephalic mass of ner- 

 vous material surrounds an oeso- 

 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 oesophagus for the purpose 

 of drawing blood through this 



B 



Fig. 26. — Transverse Section 

 through the brain of a young 

 Thelyphonus. 



exceedingly narrow tube. 



increasing 



In Fig. 25 this 

 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 



-4, supra-oesophageal ganglia; B, infra 

 oesophageal ganglia; Al, cesopkagus. 



