528 T?AY:\ioxn tkat^T;. 



siinplo oxperimont. If a disli containiiiQ^ specimens not in 

 any way stinod up by rough liandliug, but gliding along the 

 bottom, bo jaiTcd ever so slightly, every individual will 

 instantly stop, contract, and remain immovable. If only a 

 single jar is given, the worms will start after only a momentary 

 pause. A further experiment shows more strikingly the same 

 thing. If in a dish containing water to a depth of not more 

 than 1 to 1*5 cm. a single specimen gliding quietly is 

 selected, and a needle is touched to the surface of the water 

 above or to one side of it, it will be seen, if closely watched, 

 to give the same momentary pause and partial contraction. 

 If the needle is pushed down through the w^ater towards the 

 worm in any but the quietest and gentlest way the contracted 

 state will continue. Only at the moment when quietness in 

 the surroundings intervenes again will movement be resumed. 

 I have frequently tried to introduce a needle close beside the 

 animal without causing this momentary pause. With a layer 

 of water not over a centimetre in depth covering the worm I 

 have not been able to do this, except in rare instances. 

 After the point is once through the surface film it may be 

 brought nearer the worm without causing a persistence of the 

 contraction, provided it is advanced in line with itself, i. e. 

 not slid up laterally. In order to observe this extreme sensi- 

 tiveness to disturbance of the water one mnst take care that 

 the animals have not been violently disturbed just previously. 

 Any marked disturbance or persistent, more or less violent 

 stimulation puts the animals in a condition which may be 

 called, for lack of a better terra, " excited." Sucli a condition 

 is characterised by increased rapidity of movement and in- 

 creased .geHeral activity, and in this condition the animals do 

 not give the " finer " responses, — that is, responses to weak 

 stimuli. I shall have occasion to discuss this matter in more 

 detail later. 



This marked sensitivity and its associated behaviour are 

 remarkably similar to what has l)con found by Whitman ('iM)) 

 to ol)tain in the case of the leech Clepsine. lie has further 

 ])niiit(Ml out that lack ol attention to this (^xtrcnu' sensitivity. 



