142 THE POPULAR SCIENCE MONTHLY 



cular muscles on which rests a second nervous layer, Meissner's plexus. 

 Each plexus, so far as is known, is a true nervous net as intimately 

 related to the adjacent muscle fibers as is the case of the sea- anemones. 

 In fact one of the muscle layers and the adjacent plexus in the intestine 

 reproduce very accurately all the essentials of the neuromuscular mech- 

 anism of a sea-anemone except the epithelial sense-cells. 



Not only is there this anatomical similarity between the neuro- 

 muscular mechanisms of the sea-anemone and of the vertebrate intes- 

 tine, but there is also a physiological similarity as seen in the movements 

 of the digestive tube. The essentials of these movements are well 

 exemplified in the small intestine. In this part of the digestive tube 

 the characteristic movements are segmentation and peristalsis. Seg- 

 mentation consists in a series of temporary, ring-like constrictions in 

 the intestinal wall that come and go in such a way that the enlarged 

 region of the tube between any two constrictions is the site of the 

 constriction next to appear, and so on. As a result of segmentation, the 

 food is most thoroughly churned and mixed. Peristalsis is a wave-like 

 movement whereby the food is carried posteriorly through the intestine. 

 Usually these two movements go on together in such a way that the 

 peristalsis is combined with segmentation in that the latter becomes 

 somewhat unsymmetrical and cuts each food mass into two unequal 

 parts the larger of which is on the posterior side of the constriction. 

 Hence the food is not only churned but is at the same time moved pos- 

 teriorly through the intestine. 



The small intestine receives nerve-fibers from two extraneous 

 sources, the vagus and the splanchnic nerves, and it might be supposed 

 that these were essential for the movements of the intestine. But as 

 Cannon (1906) has demonstrated, both sets of nerves may be cut, and 

 yet after recovery from the immediate effects of the operation seg- 

 mentation and peristalsis will be found to go on in the digestive tube 

 in an essentially natural manner. It is thus clear that the vertebrate 

 intestine, like the tentacle of a sea-anemone, contains a complete neuro- 

 muscular mechanism within its own wall, and though there is no histo- 

 logical evidence of the presence of receptors reaching from the mucous 

 surfaces of the intestine to the nervous nets within, yet there are sound 

 physiological grounds for assuming the presence of such organs. In 

 that case the type of neuromuscular mechanism in the intestine would 

 be practically identical with that in the sea-anemone. 



A second example of a receptor-effector system in ccelenterates is 

 seen in the jellyfishes. In these animals as contrasted with the sea- 

 anemones, locomotion is a well-developed activity, and it is the neuro- 

 muscular mechanism concerned with this function that must be con- 

 sidered. The structures involved in locomotion are well exemplified 

 in Aurelia (Fig. 4). This common jellyfish possesses on the edge of 



