PHYSIOLOGY OF THE INVERTRBRATA. 301 



nature. After a variable period occasional contractions are 

 usitally given, or, in some cases, the contractions may be 

 resumed with but little apparent detriment. Considerable 

 differences, however, in these respects are manifested in 

 different species, and also by different individuals of the 

 same species. Hence, in comparing the covered-eyed group 

 as a whole with the naked-eyed group as a whole, I should 

 say that the former resembles the latter in that its repre- 

 sentatives usually have their main supply of locomotor centres 

 situated in their margins, but that it differs from the latter 

 in that its representatives usually have a greater or less 

 supply of their locomotor centres scattered through the 

 general contractile tissue of their organs. But although the 

 locomotor centres of a covered-eyed Medusa are thus, generally 

 speaking, more diffused than are those of a naked-eyed Medusa, 

 if ivc consider the organism as a whole, the locomotor centres 

 in the margin of a covered-eyed Medusa are less diffused 

 than are those in the margin of a naked-eyed Medusa. In 

 no case does the excision of the marofin of a swimrainof 

 organ produce any effect upon the movements of the 

 manubrium." 



Romanes has proved the effects of various stimuli upon the 

 Medusa:. After the removal of the locomotor centres (ganglia) 

 all these animals invariably respond to stimulation, but the 

 degrees of irritability in responding to stimuli differ con- 

 siderably in different species. 



The covered-eyed, and a few of the naked-eyed Medusa) 

 respond with one or more contractions to the action of light. 

 In the case of Sarsicc tubnlosa, a flash of light causes it to 

 respond ; in fact, light acts as a stimulus. It has been ob- 

 served that the marginal bodies of Sarsia are organs of special 

 sense, adapted to respond to luminous stimulation ; in other 

 words, they perform the function of sight — in fact, the marginal 

 bodies are rudimentary eyes. 



Romanes has shown that when these marginal bodies are 

 excised, the mutilated animals did not seek the light, " but 



