244 



BEHAVIOR OF THE LOWER ORGANISMS 



the body (Fig. 136). In this way the exposed surface of the body is 

 made as small as possible, and the sensitive head especially is kept from 

 drying. At intervals the animal straightens out, extends its head as 

 far as possible, and waves it from side to side. If in this way it finds 

 water, it of course moves into it. If it does not find water, it curls up 

 again. After a time, if the drying becomes more decided, the animal 

 attempts to crawl backward. Under natural conditions drying will 

 usually take place at the edge of a pool, and this backward movement 

 carries the animal again into the water. All together, the reaction to 

 drying is not simple and stereotyped, but involves the successive per- 

 formance of many different activities. 



In responses to heat or cold we find a similar train of activities. If 

 the gliding Planaria comes to a region of considerably higher or lower 



temperature, it waves its head back 

 and forth several times, apparently 

 till it has determined the direction 

 which leads back to the usual 

 temperature, then turns and moves 

 in that direction. Responses of 

 this character usually take place 

 several times before the animal is 

 completely directed toward the re- 

 gion of optimum temperature (see 

 Fig. 137). If the temperature of 

 the water is slowly raised in a 

 uniform manner, so that all parts 

 of the body are similarly affected, 

 then a series of reactions occurs. 

 First the animals become more 

 Fio. 137.— Behavior of the flatworm in ap- active, gliding about rapidly, ex- 

 proaching the heated end of a trough The lines tending the head, and turning it 



show the paths followed. At each of the points ° ' ° 



marked by a round spot, the animal stopped and tOWard one Side Or the Other. The 



waved its head to and fro, finally following up behavior resembles that of speci- 



one of the trial movements. The figures at these . . . , , 



points show the number of trial movements that mens showmg the pOSltlVe ' re- 

 were made, in each case. After Mast. ^ction to Weak stimuU. As the 



temperature rises, the animal begins to contract at intervals, and to turn 

 the head frequently and strongly from side to side, making little prog- 

 ress in advance. The behavior has now the characteristics of the 

 "negative" reaction. As the temperature rises further, the turning 

 ceases, and the animal begins to make rapid, violent contractions, such 

 as occur in "crawling," under other violent stimuli. Later the animal 

 twists its body, as occurs in its righting reaction when placed on its 



