PHYSIOLOGY OF NERVOUS SYSTEM. 733 



contrasted with the Craspedote forms. Sensitiveness to 

 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 muscular contraction does follow the application 

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

 of the nerve centre 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 Medusse. An interesting point is the absence of 

 the spontaneous movement which is so characteristic of the 

 Medusse. 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 jellyfish. Recently in the Great Barrier 

 reef of Australia an Alcyonarian has been found, in which 

 the polypes, though sessile, exhibit a constant rhythmic 

 contraction and expansion of their tentacles, so that the 

 tendency to exhibit continuous rhythm is widely spread 

 in the Coelentera. The nervous system is apparently 

 tolerably uniform in type throughout the group ; what 

 determines the physiological peculiarities has yet to be 

 investigated. There are two rival explanations of rhythmic 

 movements, such as those of the umbrella of the Medusae. 

 According to the first, it is caused by rhythmic stimuli, 

 passing out from the nerve centres to the muscles con- 

 cerned, and thereby causing the contractions. The other 



