220 PARKER— ORIGIN AND SIGNIFICANCE OF [April 21, 



have long been inclined to regard it as representing the first step 

 in the dififerentiation of the neuromuscular organs. This plan of 

 structure is well represented in the sea-anemones. Each of the two 

 chief layers of cells that make up the living substance of the sea- 

 anemone's body consists of three sublayers : a superficial or epithelial 

 layer, a middle or nervous layer, and a deep or muscular layer. The 

 epithelial layer contains, besides many other kinds of cells, large 

 numbers of sensory cells which terminate peripherally in bristle- 

 like receptive ends and centrally in fine nervous branches. These 

 fine branches constitute collectively the middle or nervous layer in 

 which occasionally large branching cells, the so-called ganglionic 

 cells, occur. Immediately under the nervous layer is the deep layer 

 of elongated muscle-cells. The condition thus briefly described is 

 present over the whole of the sea-anemone's body and though the 

 nervous layer is somewhat emphasized in the neighborhood of the 

 mouth, it cannot be said to be really centralised in any part. Hence 

 this type of nervous system has been designated as diffuse in con- 

 trast with the centralised type found in the higher metazoans. 



Not only is the structure of the nervous system of the sea-anem- 

 one appropriately described as diffuse, but in its action this system 

 shows those peculiarities that would be expected from the possession 

 of so diffuse an organization. Since each part of the animal con- 

 tains its own nerve and muscle, it is not surprising that after isola- 

 tion many of these parts will respond to stimuli much as they did 

 when they were a constituent of the whole organism. Tentacles, for 

 instance, when freshly cut from the body of a sea-anemone will re- 

 spond to pieces of food by encircling them, etc., in much the same 

 way as when these organs were parts of a normal animal. ]^Iuch 

 evidence of this kind has shown conclusively that the nervous sys- 

 tem of coelenterates is no more centralized physiologically than it is 

 anatomically, but is in all respects essentially diffuse. 



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, 

 i. e., without the intervention of a true central organ. This neu- 

 romuscular system, if described in the terms already used, could be 

 said to be composed of receptors and eftcctors without an adjuster 



