38 G. H. Parker 



may be appropriately called centralized. When this concentra- 

 tion takes place it usually occurs near the chief group of sense 

 organs and gives rise to what is conventionally called the brain. 

 In nervous differentiation, then, the chief central organ or brain 

 follows, in its early evolution, the lines of sensory differentiation. 

 The differentiation of the complete neuromuscular mechanism 

 as possessed by the higher animals, has occurred, I believe, in 

 three successive steps; first, the formation of independent effectors, 

 tor as seen in the muscles of sponges; secondly, the addition of 

 receptors to such effectors, as seen in what I have elsew^here called 

 the receptor-effector systems of the coelenterates; and finally, the 

 differentiation near the receptors of adjusters or central organs con- 

 cerned primarily with easy transmission from receptors to 

 effectors (Parker '09). 



6. SUMMARY 



1 Stylotella under natural conditions closes its oscula and 

 contracts its flesh when at low tide it is exposed to the air. 



2 Its outer surface is perforated by many ostia which lead to 

 large subdermal cavities, these in turn connect through incurrent 

 canals with the flagellated chambers from which excurrent canals 

 pass to the gastral cavity and the osculum. 



3 The flesh of Stylotella contains many myocytes, which are 

 arranged as sphincters around the ostia, internal cavities, and 

 osculum. These sphincters apparently work against the general 

 elasticity of the flesh and not against radiating systems of myo- 

 cytes. 



4 The oscula close in quiet seawater, on exposure to air, on 

 injury to neighboring parts, in solutions of ether (0.5 per cent), 

 chloroform (0.5 per cent), strychnine (ygioo)) cocaine (toVo)> 

 and in deoxygenated seawater. They contract but do not close 

 in diluted seawater and at temperatures higher than normal (35° 

 to 45° C). They remain open in currents of seawater, and their 

 closure is inhibited by solutions of cocaine (yoioo) and of atropine 

 (tttVo^)? and in fresh water. They are apparently uninfluenced 

 by low temperatures, by weak solutions of cocaine (^^oioo) and of 

 atropine (tttt^tt) and hy light. 



