210 BEHAVIOR OF THE LOWER ORGANISMS 



tacles contract strongly. Pieces of the animal react in essentially the 

 same way as the entire organism, and young buds (with tentacles) 

 react in the same way as adults, but are more sensitive to the current 

 (Pearl, 190 1). 



Medusa. — If strips of various shapes are cut from the medusa 

 Polyorchis, and subjected to the action of the constant current, the 

 tentacles and manubrium bend toward the cathode (Fig. 130, a, b). 

 This takes place even with isolated tentacles (Fig. 130, c). If the 

 current is long continued, such isolated tentacles partially relax, then 

 contract again. This is repeated, so that an irregular rhythmic con- 

 traction is produced by the constant current. 



B. Reactions to Gravity 



The position of the body and the direction of locomotion are partly 

 determined in some of the Ccelenterata by gravity. There is great 

 diversity among different members of the group in this respect. In 

 some, gravity is an almost constant determining factor in the behavior. 

 In others it plays only an incidental part, affecting the behavior under 

 certain circumstances, while in still other cases it seems to have no effect 

 on the movements whatsoever. 



We have already seen that the position taken by Cerianthus is partly 

 determined by gravity. The sea anemone Sagartia, according to Torrey 

 (1904), usually moves upward when this is possible, and at the same 

 time it tends to keep its body in line with gravity, with the disk above. 

 If while moving on the floor of the aquarium it reaches the perpendicu- 

 lar side, it at once begins to ascend. Since Sagartia creeps by move- 

 ments of its foot, remaining in the upright position, its ascent on a 

 vertical surface involves bringing the body into an oblique position, 

 in place of the usual perpendicular one. Thus its tendency to creep 

 upward interferes with its tendency to keep its body in line with gravity, 

 and the former prevails. Sagartia may also creep on the under side 

 of the surface film, with head down, so that it is by no means a rigid 

 requirement that the head shall be above. Doubtless many other sea 

 anemones will show a tendency to keep the body in a certain position 

 with reference to gravity. 



In the hydroid Corymorpha, according to Torrey (1904 a), there is 

 a decided tendency to take a position with the head (or oral end) upward. 

 When placed in an inverted or oblique or horizontal position, Cory- 

 morpha rights itself by a bending of the body, which is due, according 

 to Torrey, not to muscular contraction, as in the sea anemones, but to 

 a change in the turgidity of the large axial entoderm cells. Those on 



