INTEODUCTION 21 



The condition thus briefly described is present over much 

 of the sea-anemone's body, and though the nervous layer 

 may be somewhat emphasized in some regions, it cannot 

 be said to be really centralized in any part. Hence this 

 type of nervous system has been designated as diffuse in 

 contrast with the centralized type found in the higher 

 animals. 



What is really present in the neuromuscular portion 

 of the sea-anemone's body is a large number of peripheral 

 sensory cells whose deep branching ends connect more or 

 less directly with the muscles, that is, without the inter- 

 vention of a true central organ. This neuromuscular sys- 

 tem, if described in the terms already used, could be said 

 to be composed of receptors and effectors without an ad- 

 justor or at least with this member present in only a most 

 undeveloped state. Hence the adjuster or central organ 

 is in all probability an acquisition that represents a later 

 stage in the evolution of the neuromuscular mechanism 

 than that seen in the crelenterates. 



If the coelenterates represent a stage in the develop- 

 ment of- the neuromuscular mechanism in which sensory 

 cells and muscles are the only important parts present, 

 it is natural to ask if there is not a still more primitive 

 state from which the ccelenterate condition has arisen. On 

 this question several hypotheses have already been ad- 

 vanced. Glaus (1878) and, subsequently, Chun (1880) 

 maintained that originally the nervous system and the 

 muscles were differentiated independently and that they 

 became associated only secondarily. This view has de- 

 servedly received very little attention, for it is extremely 

 difficult to conceive of an animal that would develop re- 

 ceptive ability without at the same time acquiring the 

 power to react. Such an animal would have a certain 



