196 BEHAVIOR OF THE LOWER ORGANISMS 



ment for Cerianthus. Loeb found that it would not remain indefinitely 

 in this position on the wire screen, as it does in the sand. After a day or 

 so it pulls its foot out of the wire and seeks a new abode. Only when it 

 can get the surface of its body in contact with something, as is the case 

 when it is embedded in the sand in its natural habitat, is it at rest. If 

 this condition is fulfilled, the requirement of the usual position in line 

 with gravity may be neglected. Loeb found that when the animal is 

 placed in a test-tube, so that its body is in contact with the sides, it re- 

 mains here indefinitely, even though the tube is placed in a horizontal 

 position (Loeb, 1891, p. 54). The head is bent upward, but the body 

 remains transverse to the direction of gravity. Similarly, the anemone 

 Sagartia may ofttimes take a position on the surface film with head 

 down, although usually it maintains an upright position (Torrey, 1904). 



But even the usual position in line with gravity, and with sides in 

 contact, does not satisfy Cerianthus indefinitely, if left quite undisturbed. 

 If it secures no food, it again leaves its place and seeks another region. 



Thus that the animal may remain quiet in a given position a consid- 

 erable number of conditions should be fulfilled, constituting altogether 

 what we may call the "normal" state of the animal. The conditions are 

 the following: (i) the foot should be in contact; (2) the head should 

 be free ; (3) the body should be straight ; (4) the axis of the body should 

 be in fine with gravity, with the head above; (5) the general body sur- 

 face should be in contact ; (6) food should be received at intervals. 



If these conditions are largely unfulfilled, the animal becomes rest- 

 less, moves about, and finds a new position. But no one of these condi- 

 tions is an absolute requirement at all times, unless it be that of having 

 the head free. In the wire screen (Fig. 122) the animal remains for a 

 day or so if in the required position with reference to gravity, even 

 though foot and body surface are not in contact. In the horizontal test- 

 tube it remains with foot and surface in contact, though the body is not 

 straight nor in line with gravity. If all conditions are fulfilled save that 

 of food, the animal remains for a time, then finally moves away. 



Clearly, the holding of any given position depends, not on the rela- 

 tion of the body to any one or two sources of stimulation, but on the 

 proper maintenance of the natural physiological processes of the organ- 

 ism. The animal does not always maintain a certain position with rela- 

 tion to gravity, nor does it always keep its body straight, nor its foot in 

 contact, nor its body surface in contact. It does not at all times receive 

 food. It may remain for considerable periods with one or more condi- 

 tions lacking. It tends on the whole to take such a position as is most 

 favorable to the unimpeded course of the normal physiological processes. 

 Certain usually required conditions may be dispensed with, provided 



