THE RE-ACTIONS OF ORGANIC MATTER ON FORCES. 55 



leleozoa. These sensible motions of animals are effected 



by various organs under various stimuli. In the humblest 

 forms, and even in. some of the more developed ones which 

 inhabit the water, locomotion results from the vibrations of 

 cilia : the contractility resides in these waving hairs that 

 grow from the surface. Some of the AcalephtB, and their 

 allies the Polypes, move when mechanically irritated : the 

 long pendant tentacle of a Physalia is suddenly drawn up if 

 touched ; and, as well as its tentacles, the whole body of 

 a Hydra collapses if roughly handled, or jarred by some 

 shock in its neighbourhood. In all the higher animals how- 

 ever, and to a smaller degree in many of the lower, sensible 

 motion is generated by a special tissue, under the special ex- 

 citement of a neural discharge. Though it is not strictly true 

 that such animals shoi^ no sensible motions otherwise caused ; 

 since all of them have certain ciliated membranes, and since 

 the circulation of fluid in them is partially due to osmotic and 

 capillary actions ; yet, generally speaking, we may say that 

 their movements are effected only by muscles that contract 

 only through the agency of nerves. 



What special transformations of force generate these various 

 mechanical changes, we do not, in most cases, know. Those 

 re-distributions of fluid, with the alterations of form sometimes 

 caused by them, that result from osmose, are not, indeed, 

 quite incomprehensible. Certain motions of plants which, 

 like those of the " animated oat," follow contact with water, 

 are easily interpreted ; as are also such other vegetal motions 

 as those of the Touch-me-not, the Squirting Cucumber, and the 

 Carpobolus. But we have as yet no clue to the mode in which 

 molecular movement is transformed into the movement of 

 masses, in animals. "We cannot refer to known causes the 

 rhythmical action of a Medusa's disc, or that slow decrease of 

 bulk that spreads throughout the mass of an Alcyomum, when 

 one of its component individuals has been irritated. ~Nor 

 are we any better able to say how the insensible motion 

 transmitted through a nerve, crives rise to sensible motion in 



