102 SCIENCE PROGRESS. 



suppose that it is homologous with the ectodermic nervous 

 system of the Coelenterates. 



Regarding the nervous systems of these animals from 

 a general standpoint, we notice that they differ from 

 the Coelenterates in two particulars. Firstly, in the pres- 

 ence of certain ganglia are tracts of nerve fibres in addi- 

 tion to the subectodermic plexus of nerve cells and fibrils, 

 and secondly, in the fact that the endodermic nervous 

 system is either absent altogether, or else that it is so 

 poorly represented that it has hitherto escaped observa- 

 tion. (The plexus of Auerbach and Meissner in the 

 alimentary canal of Vertebrates may represent the endo- 

 dermic nervous system, but so far as I am aware there is 

 no embryological proof of this at present.) 



The predominance of the ectodermic nervous system in 

 the higher animals is readily accounted for by the fact that 

 all the most important sensory impulses must originate in 

 ectodermic tissues and the great muscles which move the 

 body and limbs of the animal are derived from the somatic 

 i.e., the subectodermic layer of the mesoblast. The 

 muscles of the splanchnic layer of the mesoblast which are 

 moved by the endodermic nervous system — if such a system 

 is really present — are comparatively slight, and the sensory 

 impulses of the endoderm, although important physio- 

 logically perhaps, are general rather than special in 

 character. 



Two important questions have now to be considered. 

 Firstly, do the plexus and the fibrils situated in the ectoderm 

 of Coelenterates really perform the same or similar functions 

 as the epiblastic nervous system of the higher animals ? 

 And secondly, is there any direct or indirect histological 

 communication between the ectodermic and endodermic 

 plexuses ? 



In the consideration of the answer to the first question 

 we must remember that the histological similarity of the 

 tissues of two animals is not a conclusive proof that they 

 perform the same functions. To prove that any system of 

 cells and fibrils is nervous in function we must have physio- 

 logical evidence that they originate, reflect, or transmit 



