144 THE INVOLUNTARY NERVOUS SYSTEM 



for the belief that the vertebrates arose without reversal of sur- 

 faces from the Palaeostraca forms which were neither Crusta- 

 ceans nor Arachnids, but gave origin to both ; forms which were 

 much more nearly allied to the worms than are the crayfish and 

 spiders of the present day ; and the resemblances which I have 

 put forward are based partly on the anatomy of such animals 

 as king crabs and sea scorpions, partly on the structure of 

 worms. 



It is therefore a striking fact that Poll and Sommer found cer- 

 tain chromaffine nerve cells, probably therefore containing adren- 

 aline, in the central nervous system of worms. Further investiga- 

 tion on this matter has been carried out by my son. He searched 

 in a large number of Annelids and Crustaceans for signs of chro- 

 maffine tissue in some organ or other, and arrived at the conclu- 

 sion, that the adrenaline is confined to certain nerve cells in the 

 central nervous system of certain Annelids. In the Crustaceans 

 examined by him he found no sign of chrome staining in any cells 

 of the central nervous system. He did not however carry the 

 investigation to tissues outside the central nervous system, there- 

 fore the presence of chromaffine cells outside the central nervous 

 system in invertebrates has not been excluded. These cells give 

 a most marked reaction, and in all Annelids investigated by him, 

 in which they occur, are invariably three in number on each side 

 of a ganglion and constant in position. He has found them in 

 the Hirudineae, Lumbricus Herculeus, Aphrodite Aculeata and 

 Eunice Gigaritia. In all cases, where he has found them, it is 

 striking to see how well developed are the muscles of the vascu- 

 lar system ; where the reaction is not to be seen, as in most of 

 the sea-living Polychaetes, true vascular muscles do not appear to 

 exist. These observations point strongly to the conclusion that 

 these three cells on each side in each ganglion have to do with 

 the innervation of vascular muscles. 



In Hirudo and Aulostoma the most ventrally situated of 

 these three cells on each side of any ganglion of the ventral chain 

 is a colossal cell, so that there are in each ganglion two of these 

 colossal cells, which are so much bigger than any other cells in 

 the ganglion that they cannot be mistaken however they are 

 stained. In all cases, where these colossal cells occur, they stain 

 with chrome salts and probably contain adrenaline. It might be 

 objected that enormous cells like these cannot send motor fibres 



