642 RAYMOND PEAKL. 



the organism turus toward all bodies of a certain physical 

 texture under most circumstances. Tlie active squeezing of 

 the material in the "gripping" undoubtedly helps to press 

 out to the surface any juices which may be in the material. 



In closing the section on food reactions it may be well to 

 give a sort of general picture of the whole behaviour of 

 fresh-Avater planarians towards food. The method by which 

 the planarian finds material suitable for food is as follows : 



1. Chemical substances diffusing from food come in con- 

 tact with the sensitive head region of the planarian ; or — 



The movino- animal touches with the head some soft sub- 

 stance, and as a result of either of these two sorts of 

 stimulation — 



2. The organism gives a positive reaction, i. e. turns 

 towards the source of the stimulus. This reaction is very 

 precisely localised in most cases, and is the most essential 

 part of the whole food behaviour. Its mechanism has been 

 previously described (v. sup., p. 619). 



3. When the anterior end squarely touches the food as a 

 result of this reaction it typically closes tightly over it, 

 giving what I have called the ''gripping'^ reaction. This 

 reaction is evidently a very much specialised feeling move- 

 ment for the purpose of closely testing the chemical nature 

 of material. It is produced by a contraction of the ventral 

 longitudinal muscles of the head region. While it is taking 

 place progressive motion ceases. 



4. After this pause the worm glides over the piece of food 

 till the opening of the pharyngeal sac lies over or nearly 

 over the place " gripped/' and there the posterior part stops 

 and the pharynx is extruded and attached to the food. The 

 factors determining the place where the pharynx shall be 

 extruded are (a) the stimulation of the ventral surface of 

 the body in the pharyngeal region of the food (pure reflex 

 factor), and [h) the presence of the brain, which probably 

 acts as a co-ordinating centre for this reaction. 



•5. A digestive Huid is poured out through the pharynx, 

 and the food is partly digested before being taken up. 



