MOVEMENTS, ETC., OF EIJESlI-WATEl; I'LANAEIAMS. 625 



been placed iu the dish specimeus may pass very near it — 

 within two or three luilhmetres — without being affected in 

 any way. They simply glide straight by as if there were no 

 food there. After a few minutes have passed, however, it 

 will be found that a worm coming near the food is affected 

 in a very characteristic manner. Its behaviour is as 

 follows: — When within about three or four millimetres of 

 the piece of meat (Fig. -0, o) it stops abruptl}', raises the 

 head, and turns it towards the food (Fig. 2G, h). As the head 

 is raised and turned the gliding is resumed, and the head 

 being almost immediately lowered, the movement is directly 

 towards the food. Thus fur the reaction is evidently 

 precisely like the positive reaction to weak mechanical 

 stimuli, and so we may speak of it as the positive reaction to 

 food, the reaction being the same in the two cases, though 

 the stimulus differs. ^Vhen the anterior end of the head 



Tig. 27. — Diagrammalic siiie view of Planaria to show tlie 

 *' gripping " ol' a bit of fooil, A. 



touches the food it flattens down upon it, and, if the con- 

 figuration is such as to nnike it possible, " grips " it 

 (Fig. 26, c). The details of this " gripping " (shown in side 

 view in Fig. 27) are as follows : — The anterior end closes 

 down over the very edge of the piece of food, or over the 

 whole piece provided it is small enough, and then 

 apparently squeezes it by contraction of the longitudinal 

 muscles on the ventral surface of the head, 'i'lie action is 

 very characteristic, and evidently forms an integral part of 

 the normal food reaction. Its probable function will be 

 brought out Inter. While it is taking place the worm as a 

 whole stops its progressive movement and remains quiet. 

 After the " gripping" has continued for some time the worm 

 starts gliding ahead up on to the food. It passes forward 

 till the point where the opening for the extrusion of the 

 pharynx is located is approximately over the place pre- 



