376 PHYSIOLOGICAL DEVELOPMENT. 



and the rapidity with which such a local stimulus is re- 

 sponded to by a more-differentiated animal. A sea-anemone 

 and a fly will serve for the comparison. A tentacle of a sea- 

 anemone, when touched, slowly contracts; and if the touch 

 has been rude, the contraction presently extends to the other 

 tentacles and eventually to the entire body: the stimulus to 

 movement is gradually diffused throughout the organism. 

 But if you touch a fly, or rather if you come near enough 

 to threaten a touch, the entire apparatus of flight is instantly 

 brought into combined action. Whence arises this contrast? 

 The one creature has but faintly specialized contractile 

 organs, and fibres for conveying impressions. The other has 

 definite muscles and nerves and a co-ordinating centre. The 

 parts of the little-differentiated sea-anemone have their 

 functions so feebly co-ordinated, that one may be strongly 

 affected for some time before any effect is felt by another at 

 a distance from it ; but in the much-differentiated fly, various 

 remote parts instantly have changes propagated to them from 

 the affected part, and by their united actions thus set up, the 

 whole organism adjusts itself so as to avoid the danger. 



These few added illustrations will make the nature of this 

 general relation sufficiently clear. Let us now pass to the 

 interpretation of it. 



307. If a Hydra is cut in two, the nutritive liquids 

 diffused through its substance cannot escape rapidly, since 

 there are no open channels for them; and hence the con- 

 ditions of the parts at a distance from the cut is but little 

 affected. But where, as in the more-differentiated animals, 

 the nutritive liquid is contained in vessels which have con- 

 tinuous communications, cutting the body in two, or cutting 

 off any considerable portion of it, is followed by escape of 

 the liquid from these vessels to a large extent; and this 

 affects the nutrition and efficiency of organs remote from 

 the place of injury. Then where, as in further-developed 

 creatures, there exists an apparatus for propelling the blood 



