58 THE DATA OF BIOLOGY. 



trast, 3^et, on comparing the quantities of matter propelled 

 through given spaces in given times, they will see that the 

 momentum evolved is far less in the Protozoa than in the 

 Metazoa. These sensible motions of animals are 



effected in sundry ways. In the humblest forms, and even 

 in some of the more developed forms which inhabit the 

 water, locomotion results from the oscillations of whip-like 

 appendages, single or double, or from the oscillations of 

 cilia: the contractility resides in these waving hairs that 

 grow from the surface. In many Coelenterata certain elonga- 

 tions or tails of ectodermal or endodermal cells shorten when 

 stimulated, and by these rudimentary contractile organs the 

 movements are effected. In all the higher animals, however, 

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

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

 citement. Though it is not strictly true that such animals 

 show no sensible motions otherwise caused, since all of them 

 have certain ciliated membranes, and since the circulation 

 of liquids in them is partially due to osmotic and capillary 

 actions ; yet, generally speaking, we may say that their move- 

 ments are effected solely by muscles which contract solely 

 through the agency of nerves. 



What special transformations of force generate these vari- 

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

 Those re-distributions of liquid, with the alterations of form 

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

 indeed, 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 Carpoholus. But we are ignorant of 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 which spreads throughout the mass of an Alcyonium 

 when one of its component individuals has been irritated. 



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