776 COMPARATIVE PHYSIOLOGY. 



light is exhibited in the same way as in the latter. The 

 central nervous system is connected by a nerve-plexus with 

 the muscles which effect movement. Although little is 

 known histologically of the way in which the nerves end in 

 the muscles, yet physiologically, in its relation to poisons, the 

 peripheral termination shows a remarkable resemblance to 

 the "end-plate" which characteristically occurs in the 

 muscles of Vertebrates. We find here, therefore, even at 

 this low stage, that the three distinct parts of a nervous 

 system are quite clearly defined. It seems unlikely that 

 division of labour has gone so far as to definitely differ- 

 entiate sensory and motor nerves, but it is important to 

 note that, as in higher forms with distinct afferent and 

 efferent nerves, muscular contraction follows the application 

 of a stimulus. The difference as to the effect of the removal 

 of the nerve-centres in the two types is extremely interesting, 

 but as yet unexplained. 



In sea-anemones the nervous system has been less fully 

 investigated than in the Medusae. There are no specialised 

 nerve-centres : nearly all parts of the body when separated 

 seem to be able to respond to stimuli, so that the nerve- 

 cells must be scattered. The relation of the muscles to 

 the nervous tissue has the same physiological complexity 

 as in the Medusae. An interesting point is the absence of 

 the spontaneous movement which is so characteristic of the 

 Medusae. We have the same contrast often presented even 

 in the life history of the individual, compare the sessile 

 hydroid and the active swimming-bell, the fixed hydra-tuba 

 and the pelagic jelly-fish ; but the cause of the difference is 

 as yet unknown. There are two rival explanations of 

 rhythmic movements, such as those of the umbrella of the 

 Medusae. According to one, they are due to rhythmic 

 stimuli passing out from the nervous centres to the muscles 

 concerned, and thereby causing the contractions. The 

 other view is that the regular contractions are due to the 

 activity of the muscles themselves. On this hypothesis, 

 building-up processes go on in the muscles until extremely 

 unstable substances are produced ; these explode and break 

 down into simpler compounds, the process being accom- 

 panied by an evolution of energy manifested by the contrac- 

 tion of the muscle. The process, repeated at regular 



