164 TEXTBOOK OF ZOOLOGY 



animals are rather gregarious and when at rest will group together 

 beneath objects where the light is not intense. They respond nega- 

 tively to bright light. They usually feed upon minute plants and 

 animals, dead animal bodies, and living forms, such as small arthro- 

 pods and molluscs. Planaria partially encompasses the food with 

 the body, while the pharynx is protruded to eat it. If tiny scraps 

 of meat are placed in a dish with hungry planarians, they will form 

 a wad of living protoplasm about it. The mouth is located at the 

 middle of the ventral side of the body, and the pharynx is everted 

 through it as a prohoscis which is used to draw food within. It is 

 interesting to watch these animals passing the proboscis about over 

 the surface of fresh meat, apparently sucking up the nourishing 

 fluids from the meat. If very minute quantities of meat juice are 

 liberated in the water at specific points, the planarians are at- 

 tracted to those points. 



Eye Genital pore 



Side of head 



PharjTix sheath Proboscis 



Fig. 81. — Entire planaria with pharynx extended in position for feeding. (From 

 Hegner, College Zoology, published by The Macmillan Company, after Shipley and 

 McBride.) 



The locomotion is accomplished in an easy gliding fashion by the 

 action of the beating cilia and muscular contractions of the body. 

 The ability to move along in this way is enhanced by the secretion 

 of slippery mucus which essentially lays a smooth track for the 

 moving animal. It glides over a surface, even the under side of the 

 surface film of water, and adjusts itself easily to any irregularities 

 because of the soft, flexible nature of the body. The ciliary action 

 and muscular contractions are both rhythmic and progress in waves 

 from anterior to posterior. 



The behavior of this animal is of a reflex or automatic type. The 

 receiving or receptor sensory cell transfers the impulse produced by 

 a stimulus to a ganglion cell or adjustor in the central nervous system 

 which in turn transmits an impulse to an efferent cell carrying it to 

 a muscle or gland. The planarians respond to several tropisms. They 

 possess negative phototropism and thermotropism (as regards high 



