SECTION II. RECEPTOR-EFFECTOR 



SYSTEMS 



CHAPTER VI 



THE NEUROMUSCULAR STRUCTURE OF 

 SEA-ANEMONES 



SPONGES are animals in whose structure a very sim- 

 ple type of muscle is the only part that represents the 

 neurornuscular mechanism of the higher animals. These 

 muscles, moreover, are so insignificant in amount and so 

 slight in their action that a living sponge seems more 

 like a plant than an animal in its inertness. Compared 

 with such sluggish responses as those shown by sponges, 

 the movements of hydroids, coral animals, sea-anemones, 

 jellyfish and other coelenterates are quick, though the 

 movements of these animals are in turn slow compared 

 with those of vertebrates and especially of insects. This 

 quickened rate of response, which distinguishes the 

 coelenterates from the sponges, is associated with the fact 

 that the crelenterates possess not only muscles but also 

 nervous organs in the form of simple sensory surfaces by 

 which their muscles are called into action more quickly 

 than they would be by direct stimulation. Such a sys- 

 tem includes two* of the three elements already pointed 

 out as essential to a complete neuromuscular organiza- 

 tion and may be designated from the particular ele- 

 ments present a receptor-effector system. 



The receptor-effector system with some of its most 

 important modifications is well shown in such animals as 

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