114 THE ELEMENTARY NERVOUS SYSTEM 



the two the region of the nerve-net alone is rendered rela- 

 tively more sensitive than that composed of nerve-net and 

 muscle. Bethe, therefore, concluded that, though the 

 muscle may be open to direct stimulation, it is not so sen- 

 sitive as the nerve-net and that consequently, when a por- 

 tion of a Rliizostoma without a marginal body is made to 

 contract, it probably does so through the action of the 

 stimulating agent on the nerve-net rather than on the 

 muscle. From this standpoint the neuromuscular mechan- 

 ism of a jellyfish may be regarded as composed of three 

 parts of graded susceptibility to stimulation: the mar- 

 ginal bodies, which are most sensitive and are stimulated 

 by ordinary environmental changes ; the nerve-net, which 

 is less sensitive than the marginal bodies and is only 

 rarely open to stimulation from the environment; and, 

 finally, the muscle, which is the least sensitive of the 

 three and is consequently only most rarely stimulated 

 directly by environmental changes. 



This contrast in the sensitivity of muscular and nerv- 

 ous components in the neuromuscular organization of the 

 jellyfishes is quite in accord with what should be expected 

 in the evolution of a receptor-effector system. The 

 muscles of sponges are, in comparison with those in most 

 other animals, extremely inert to stimulation and very 

 sluggish in response. In this respect they are like the 

 muscle of the coelenterate divested of its nervous connec- 

 tions. It is only after such effectors as these have added 

 to them receptors, such as are seen in the sensory sur- 

 faces of the sea-anemones or in the marginal bodies of 

 the jellyfishes, that a quicker and more efficient system is 

 established, whose improvement over the old one is de- 

 pendent upon the increased sensitiveness of the new mem- 

 ber, the receptor, rather than on any considerable change 

 in the original member, the effector. 



