BEHAVIOR OF C(ELENTERATA 199 



bend at this point (Fig. 124). Such a bending is produced by most 

 strong chemicals; strong acids placed in a capillary tube, the tip of 

 which is appKed to the body, show it clearly. As a result of the bend 

 the head of the animal becomes directed toward the chemical or the 

 heated region, and is therefore strongly stimulated, so that the Hydra 

 now contracts as a whole. Thus the result of the bending is to carry 

 the most sensitive part of the animal into the injurious agent, where it 

 is still further injured. This reaction is produced only by strong, inju- 

 rious agents, and is really an incidental result of the local injury pro- 

 duced. The point injured remains contracted for a long time after the 

 stimulating agent has ceased to act. The Hydra may contract com- 

 pletely, so that the bend disappears, but on extension the bend is still 

 found at the injured spot. It is evident that this bending reaction is 

 not a regulatory one, and it is apparently never shown in nature, since 

 the conditions necessary for its production are practically never present. 

 It is a product of the laboratory. As we shall see later, after reaction 

 in this manner. Hydra usually sets in operation other reactions, which 

 do act in a regulatory way. 



Sea Anemones. — Intense local stimulation of the column in the sea 

 anemones usually produces a contraction of the entire body, or a move- 

 ment of tentacles on the side stimulated, in the way described later. In 

 Sagartia (Torrey, 1904, p. 208), stimulation of the edge of the foot 

 induces a local contraction of the foot and base of the column, with 

 discharge of acontia — the defensive weapons of the animal. 



Local stimulation of the tentacles causes in the different sea anem- 

 ones various reactions. Often slight local stimulation causes the tenta- 

 cles to wave about; this and similar phenomena will be described in 

 connection with the food reactions. In most sea anemones local stimu- 

 lation of the tentacles, especially if intense, causes them to shorten by con- 

 traction, or to collapse and become very slender. This is followed in 

 many cases by a contraction of the whole body. In Aiptasia an immedi- 

 ate contraction of the entire body follows even a sHght stimulation of 

 the tip of one of the long tentacles. 



MeduscE. — In niedusae, intense stimulation of one side of the bell 

 •causes immediate contraction of that side, accompanied by a less marked 

 contraction of the remainder of the bell. The stronger contraction on 

 the side stimulated turns the animal away from that side, and its subse- 

 .quent locomotion removes it at once from the stimulating agent. Thus 

 the appropriate direction of movement is here determined in the sim- 

 plest way — by contraction of the part stimulated. Such effects are 

 produced by mechanical and chemical stimulations, by heat, by elec- 

 tricity, and apparently by light. In Hydra, as we have seen, identically 



