500 



EVOLUTION OF ANIMALS 



Part V 



Pharynx- 

 Mouth 



■:m 





N • 



Fig. 25.2. Habit sketches of a common fresh-water planarian. Dugesia tigrina 

 (Planaria maculata) . 1, full grown, about three quarters of an inch long, 2, feed- 

 ing on the water-soaked body of a dead fly, its sucking pharynx extended through 

 a soft place in the insect's abdomen. The eggs of planarians are usually pro- 

 tected in cocoons, about the size of pinheads, and attached to submerged rocks 

 and leaves. 



and quiet streams, its dark body so soft that it can be cut with the edge of a 

 leaf. 



A Pioneer Head. Planarians are pioneers not only in their bilateral sym- 

 metry, but in having two uniquely different ends to their bodies, one of which 

 is a recognizable head. Dugesia cannot be credited with a neck, yet the head 

 is clearly set off from the rest of the body. It bears the eyes and many sensory 

 cells, and holds the brain. As with cats and other more astute animals, the 

 head of the planarian always arrives first. It is lifted slightly and bent from 

 side to side testing and exploring the way, a faint foreshadowing of the wise 

 end of a cat. On its surface are many microscopic pits containing cells that are 

 sensitive to chemical substances. On the pointed flaps, fancifully called auricles, 

 similar chemical perceptors are set in ciliated troughs. Planarians are attracted 

 by such foods as snail blood or crushed earthworms and in a dish of water 

 they will follow a capillary tube that contains them. If certain very weak acids 

 are used as bait, a planarian will grip the tube that holds them and push its 

 pharynx up into it as if the acid were a choice flavor, comparable to grape- 

 fruit and pickles to human taste. On the other hand, when certain other sub- 

 stances are offered, it will turn sharply away as if in a hurry. With one auricle 

 removed it travels in circles in a dish of dilute snail blood, curving its body 

 toward the unhurt side by which it still responds to the attractive blood. If 

 its brain is removed it will not react even to the most desirable snail juice. 



The whole body surface will respond to a delicate touch though the head is 



