64 THE O RIG IX OF VERTEBRATES 



A review of the animal kingdom as a whole leads to the conclusion that the 

 upward development of animals from an original coelenterate stock, in which 

 the central nervous system consists of a ring of nervous material surrounding 

 the mouth, has led. in consequence of the elaboration of the central nervous 

 system, to a general plan among the higher groups of invertebrates in the topo- 

 graphical arrangement of the important organs. The mouth is situated ventrally. 

 and leads by means of the oesophagus into an alimentary canal which is situated 

 dorsally to the central nervous system. Thus the oesophagus pierces the central 

 nervous system and divides it into two parts, the supra-oesophageal ganglia 

 and the infra-cesophageal gangdia. This is an 'almost universal plan among 

 invertebrates, but apparently does not hold for vertebrates, for in them the 

 central nervous system is always situated dorsally and the alimentary canal 

 ventrally, and there is no piercing of the central nervous system by an oesophagus. 



Yet a remarkable resemblance exists between the central nervous system of 

 the vertebrate and that of the higher invertebrates, of so striking - a character as 

 to compel one school of anatomists to attempt the derivation of vertebrates 

 from annelids. Now, the central nervous system of vertebrates forms a hollow 

 tube, and a diverticulum of this hollow tube, known as the infundibulum, passes 

 to the ventral surface of the brain in the very position where the oesophagus 

 would have been if that brain had belonged to an annelid or an arthropod. 

 This school of anatomists therefore concluded that this infundibular tube 

 rejn'esented the original invertebrate oesophagus which had become closed and 

 no longer opened into the alimentary canal owing to the formation of a new 

 niouth in the vertebrate. As, however, the alimentary canal of the vertebrate 

 is ventral to the central nervous system, and not dorsal, as in the invertebrate, 

 it follows that the remains of the original invertebrate mouth into which the 

 oesophagus (in the vertebrate the infundibular tube) must have opened must be 

 searched for on the dorsal side of the vertebrate ; and so the theory was put 

 forward that the vertebrate had arisen from the annelid by the reversal of 

 surfaces, the back of the one animal becoming the front of the other. 



The difficulties in the way of accepting such reversal of surfaces have proved 

 insuperable, and another school has arisen which suggests that evolution has 

 throughout proceeded on two lines, the one forming - g - roups of animals in which 

 the central nervous system is pierced by the food-channel and the gut therefore 

 lies dorsally to it, the other in which the central nervous system always lies 

 dorsally to the alimentary canal and is not pierced by it. In both cases the 

 highest products of the evolution have become markedly segmented animals, in 

 the former, annelids and arthropods ; in the latter, vertebrates. The only 

 evidence on which such theory is based is the existence of low forms of animals, 

 known as the Enteropneusta, the best known example of which is called 

 BalauiHjlossiis ; they are looked upon as aberrant annelid forms by many 

 observers. 



This theoiy does not attempt to explain the peculiarities of the tube of the 

 vertebrate central nervous system, or to account for the extraordinary resemblance 

 between the structure and arrangement of the central nervoiis systems of 

 vertebrates and of the highest invertebrate group. 



Neither of these theories is satisfactory or has secured universal acceptance. 

 The problem must be considered entirely anew. What are the g - uiding principles 

 in this investigation ? 



