i22 TEE POPULAR SCIENCE MONTHLY 



separates the digestive cavity from the outer sea-water, and that is 

 drawn out in processes to form the hollow tentacles. In no part of 

 its structure is the sea-anemone massive, as is the case in most higher 

 forms, where the muscles, skeleton and so forth usually give rise to a 

 considerable thickness of tissue; in fact, the animal exhibits no well- 

 defined organs except the digestive organs, and may be described as a 

 membranous digestive sac. 



Although the body of the sea-anemone is really nothing more than 

 membranous walls, these walls have long been known to contain both 

 nerve and muscle. These two tissues occur over almost the whole 

 animal. According to the Hertwigs, the nervous tissue is more 

 abundant in the neighborhood of the mouth than elsewhere, and this 

 region has been regarded by some investigators as a central nervous 

 organ. But the studies of Jordan and others have shown conclusively 

 that this opinion is not correct, and that the removal of this region 

 interferes in no serious way with the reactions of the animal. Appar- 

 ently each part of the sea-anemone carries with it its own neuromuscular 

 mechanism, a condition well illustrated by the tentacles. These organs 

 are chiefly concerned with appropriating the food and are stimulated 

 by the dissolved materials in the food. A tentacle when cut off from a 

 sea-anemone and held in sea-water can still be stimulated by food and 

 will exhibit almost exactly the same kind of movements when thus 

 isolated that it did when a part of the whole animal, thus demonstrating 

 the completeness and independence of its own neuromuscular mechan- 

 ism. Nervous transmission can be accomplished from almost any part 

 of the sea-anemone to almost any other part, but as such experiments 

 as those with the tentacles indicate, no one part of the animal's nervous 

 organization seems to be more important than any other part. In 

 other words, the nervous system in the sea-anemone is diffuse rather 

 than centralized. 



When the minute organization of the nervous system of these 

 animals is studied, it is found to consist of a vast number of sensory 

 neurones which connect the surface of the animal with the underlying 

 muscles and which form there what appears to be an intricate nervous 

 network. This nervous mechanism is concerned primarily with the re- 

 ception of stimuli and the immediate excitation of the muscles. The 

 nervous mechanism is a receptor mechanism that acts as a trigger for set- 

 ting off the muscle. The whole neuromuscular apparatus seems to be 

 made up of those two elements which in the higher animals were desig- 

 nated receptors and effectors and without the intervention of an adjustor 

 or central nervous organ. Viewed from the standpoint of development, 

 this condition points indubitably to the conclusion that the central nerv- 

 ous organs were evolved only after the appearance of sense organs and 

 muscle, and that such animals as the sea-anemone may well be taken 



