432 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



the tentacle is stimulated we observe that these 

 threads are expelled, and that they are barbed ; it will 

 be within the knowledge of most of us that these 

 thread cells, as found in jelly-fishes, are efficient organs 

 of offence. Their relation to the trigger-like process 

 suggests that these projections are the first to feel the 

 pressure of any foreign body, and that the pressure 

 communicated by them to the thread-cell or nemato- 

 cyst, results in the projection of the contained thread. 



Here, then, we seem to have 

 the earliest and simplest 

 kind of automatic tactile 

 organ including, of 

 course, in the term touch 

 the general sensation of 

 pressure from without. It 

 is, at the same time, neces- 

 sary to observe that, al- 

 iss.-Teutacie of Hydra, though these trigger-hairs 



showing the $ Trigger-cell and appear to be the simplest 

 Nematocyst. (After F. E. *. 



Schuitze.) sense organs of a multicel- 



lular or metazoic animal, yet 



that some of the unicellular Protozoa are not without 

 organs of offence that are physiologically comparable to 

 the threads of thread cells, for, if we add a drop of iodine 

 to the water in which a Paramo3cium is swimming, we 

 find that it immediately thrusts out from its body fine 

 stiff processes. If, then, these are comparable to the 

 threads of a hydroid, it is clear that, functionally also, 

 the ectosarc of an infusoriaii is comparable to the 

 sensory parts of the epithelium of a hydroid, and is, 

 like it, capable of responding to definite external 

 stimuli in a definite way. It is important to observe 

 that the first indication of tactile organs is associated 

 with the protection of the individual, as much as with 

 the function of paralysing the prey which is seized 

 upon for food. As the sensory cells remain superficial 



