THE ORIGIN OF THE NERVOUS SYSTEM 



M3 



Fig. 4. 



its bell eight clusters of sense-organs. Each cluster contains an ocellus, 

 two sensory pits that are probably concerned with the chemical sense, 

 and a sense-club which may be a pressure organ. The sensory portions 

 of all these organs are modified ectoderm and from these portions nerve- 

 fibers pass out as radiating bundles to the ectoderm of the subumbrellar 

 surface. Here they merge into a nervous net which overlies the ecto- 

 dermic musculature as in the 

 sea-anemones. This muscula- 

 ture forms a circular sheet con- 

 centrically disposed with refer- 

 ence to the symmetry of the 

 jellyfish. When the bell of an 

 Aurelia is pulsing, the move- 

 ment is carried out by the more 

 or less general contraction of 

 this circular band of muscle, 

 which is brought back to its 

 original position on relaxation 

 by the elasticity of the gelat- 

 inous mass of the bell. The 

 locomotor muscle, then, is a 

 gigantic sphincter that works 

 against an elastic resistance. 



The significance of the various parts of the neuromuscular mechan- 

 ism in such an animal as Aurelia can be determined by experiment. If 

 the eight sense-bodies are removed, the animal will no longer pulse 

 spontaneously, though its muscles may be made to contract by direct 

 stimulation. If all but one sense-body are removed, the bell will pulse 

 with regularity and by artificially stimulating the single remaining body 

 a wave of muscular contraction can be sent over it. It is therefore 

 evident that the sense-bodies act like extremely delicate triggers and 

 thus touch off the contractile mechanism. In this respect, then, the 

 jellyfish is more highly developed than the sea-anemone, for the latter 

 possesses no such specialized and delicate receptors. 



The wave of contraction that passes over a bell when one of its 

 sense-bodies is stimulated, may be either a purely muscular phenomenon 

 or may be the result of nervous transmission through the nervous net 

 whereby one region after another of the musculature is brought into 

 action. The fact that this wave is not checked when the bell is cut 

 even in a most irregular way provided the subumbrellar epithelium is 

 still continuous, favors the nervous rather than the muscular interpreta- 

 tion. But stronger evidence on the nervous side than this has come 

 from an entirely different direction. Mayer (1906) has shown that the 

 subumbrellar epithelium of Cassiopea after removal will readily regen- 



Aurelia, subumbrellar surface ; s, clus- 

 ter of sense-organs. 



