EXPERIMENTS ON ARTHROPODS 109 



separated from each other, those that are connected 

 nervously always keep their gills in the same phase of 

 activity. Were there a centre of coordination in any 

 ganglion, a group of ganglia separated from this centre 

 would be active in an uncoordinated manner, but 

 such is never the case. 



3. In higher animals, the conditions controlling re- 

 spiration scarcely differ from those in Limulus. There 

 is a series of segmental ganglia in the thoracic portion 

 of the spinal cord which sends nerves to the thoracic 

 respiratory muscles of the respective segments. These 

 ganglia extend into the cervical portion of the spinal 

 cord, and the fourth, third, and fifth pairs of spinal 

 nerves give rise to the fibres of the phrenic nerve 

 which goes to the diaphragm. The diaphragm in 

 reality belongs to the corresponding segments of the 

 neck portion, and has attained its present position only 

 through a shifting of position during growth. One 

 would expect in text-books of physiology to find the 

 phenomena of respiration explained as follows : 

 Chemical changes which are continually going on in 

 the body, or in these segmental ganglia, under the 

 influence of heat (the temperature of the body), pro- 

 duce a periodic activity in these ganglia and conse- 

 quently in the respiratory muscles. The segmental 

 connection existing between the ganglia and the 

 muscles would bring about coordination just as it 

 does in Limulus. But in the majority of text-books 

 we find statements of the following character : The 

 automatic activity of the respiratory muscles is pro- 



