368 THYSIOLOGICAL DEVELOPMENT. 



responds to a stimulus applied to one of its parts, and the 

 rapidity with which such a local stimulus is responded to by 

 a more-differentiated animal. A Poh'pe and a Polyzoon, two 

 creatures somewhat similar in their outward appearances but 

 very unlike in their internal structures, will serve for the 

 comparison. A tentacle of a Polype, 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 tentacle of a 

 Polyzoon, or slightly disturb the water near it, the whole 

 cluster of tentacles is instantl}^ withdrawn, along with the 

 protruded part of the creature's body, into its sheath. AVhence 

 arises this contrast ? The one creature has no specialized 

 contractile organs, or fibres for conveying impressions. The 

 other has definite muscles and nerves. The parts of the 

 little-differentiated Polype have their functions so feebly co- 

 ordinated, that one may be strongly affected for a long time 

 before any effect is felt by another at a distance from it ; but 

 in the raore-diff*erentiated Polyzoon, 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 condi- 

 tion 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 that 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 



