THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM II 



nervous masses are of necessity ventral to the digestive tube, because 

 the mouth of the ccelenterate is on the ventral side. The striking 

 characteristic, then, of the invertebrate kingdom is the situation of a 

 large portion of the central nervous system ventrally to the alimentary 

 canal and the piercing of the nervous system by a tube — the oeso- 

 phagus — leading from the mouth to the alimentary canal. The 

 equally striking characteristic of the vertebrate is the dorsal position 

 of the central nervous system and the ventral position of the ali- 

 mentary canal combined with the absence of any piercing of the 

 central nervous system by the oesophagus. 



So fundamentally different is the arrangement of the important 

 organs in the two groups that it might well give rise to a feeling of 

 despair of ever hoping to solve the problem of the Origin of Verte- 

 brates; and, to my mind, this is the prevalent feeling among 

 morphologists at the present time. Two attempts at solution have 

 been made. The one is associated with the name of Geoffrey St. 

 Hilaire, and is based on the supposition that the vertebrate has 

 arisen from the invertebrate by turning over on its back, swimming 

 in this position, and so gradually converting an originally dorsal 

 surface into a ventral one, and vice versa ; at the same time, a new 

 mouth is supposed to have been formed on the new ventral side, 

 which opened directly into the alimentary canal, while the old 

 mouth, which had now become dorsal, was obliterated. 



The other attempt at solution is of much more recent date, and is 

 especially associated with the name of Bateson. It supposes that 

 bilaterally symmetrical, elongated, segmented animals were formed 

 from the very first in two distinct ways. In the one case the diges- 

 tive tube pierced the central nervous system, and was situated dorsally 

 to its main mass. In the other case the segmented central nervous 

 system was situated from the first dorsally to the alimentary canal, 

 and was not pierced by it. In the first case the highest result of 

 evolution led to the Arthropoda ; in the second case to the Vertebrata. 



Neither of these views is based on evidence so strong as to cause 

 universal acceptance. The great difficulty in the way of accepting 

 the second alternative is the complete absence of any evidence, either 

 among animals living on the earth at the present day or among those 

 known to have existed in the past, of any such chain of intermediate 

 animal forms as must, on this hypothesis, have existed in order to 

 link together the lower forms of life with the vertebrates. 



