CANAL OF CENTRAL NEBVOCFS SYSTEM OF VERTEBRATA. 163 



To return to the main question, what is the function of the 

 neural canal at this stage ? It seems to me that that function 

 must have been in the main a respiratory one. The water 

 entered the canal by the anterior pore, was driven through it 

 by the cilia, and at the hind end passed through the neurenteric 

 canal into the alimentary canal, and so out by the anus. In 

 support of this I appeal to certain well-known physiological 

 and anatomical facts. 



In the first place, in the Vertebrata the brain requires more 

 oxygen for its well-being than any other tissue of the body, 

 and in those Vertebrates, e.g. Amphibia and Sauropsida, in 

 which there is only one ventricle, special arrangements are 

 present to ensure a supply of pure arterial blood to the head. 

 In the second place, in the tracheate animals the central 

 nervous system has a specially rich supply of tracheae. Finally 

 in certain worms, e.g. Nemertines, Aphrodite, the whole 

 nervous system contains haemoglobin, which may be supposed 

 to exercise a special attraction for oxygen, and hold it in a 

 convenient state for the use of the nerve-cells. 



It is interesting to notice here that in most of the animals I 

 have just mentioned, in which there are special arrangements 

 for the respiration of the nervous system, the vascular system 

 is but little developed. It seems probable that the ancestral 

 Vertebrate with the siphon stage of neural canal was without 

 a well-developed vascular system. When this and definite 

 respiratory organs became developed, a new stage in the 

 evolution of the neural canal was reached, in which it lost its 

 respiratory function, this being assumed by the vascular 

 system. As a result of this, the anterior and posterior openings 

 became closed. This brings us to the present condition of a 

 closed central canal, whose function it is not within my province 

 to examine. 



Before concluding I wish to point out that a canal leading 

 into the centre of the central nervous system is not confined 

 to the Vertebrata. Such canals are present leading into the 

 cephalic ganglia of adult Nemertines, and in the most con- 



