THE NERVOUS SYSTEM OP THE LOBSTER. 323 



Motor and sensory roots are now recognised throughout the verte* 

 brate kingdom (with the exception of the Ascidians) ; they are now 

 even supposed to be present in Amphioxus, and it is a curious point 

 if this condition is confined to them alone and does not exist in any 

 invertebrate. 



Concerning decussation, I believe complete crossing of the nerves 

 has been found to exist in every vertebrate in which this point has 

 been investigated. Now from the experiments described above, — 

 especially experiment B, — it appears that although there is no marked 

 decussation, yet partial decussation does exist in the lobster, at any 

 rate functionally, contrary to the opinion of Yung ; but whether this 

 is a decussation of nerve fibres only, or whether nerve cells are con- 

 cerned in it, is not proved. This is of interest as affording a possible 

 clue to the condition found in vertebrates in which the crossing is 

 complete and in which nerve cells are concerned. 



My thanks are due to Dr. Michael Foster and to Professor A. Milnes 

 Marshall, who have kindly examined the results of my experiments 

 and also suggested valuable alterations in the paper. 



