CORRELATION BETWEEN METABOLIC GRADIENTS. 339 



3. Galvanotaxis. When placed in the electric current, Planaria 

 maculata orients in a very definite manner. The animal curves 

 into a U-form, lying upon one side, anterior and posterior ends 

 and likewise ventral surface being directed toward the cathode, 

 middle and dorsal surface directed toward the anode. Twenty 

 specimens were tested, all of which exhibited essentially the same 

 behavior. The cathodic orientation of the anterior end is more 

 marked than that of the posterior end, and the former is generally 

 in advance of the latter. The animals often assume the posture in 

 question at intervals, hold it for a short time, and between such 

 postures wander about the pan without showing any definite ori- 

 entation to the current ; in other cases the posture was held as long 

 as the current passed. It is evident that there is a remarkable 

 correspondence between the galvanotactic orientation of this species 

 and the double metabolic gradient described above. The electrical 

 gradient, in as far as the data go, also corresponds. 



The orientation of the polyclad in the current was very similar 

 to that of Planaria maculata. After some preliminary contortions, 

 all of the specimens tested turned their anterior ends toward the 

 cathode, often assuming a U-shaped posture. If the animal is 

 facing the cathode when the current is made, it remains in that 

 position, and may curve the tail under the body so that the poste- 

 rior end of the tail faces the cathode. If the animal is facing the 

 anode when the current is made, it curves its anterior end under 

 the body so that head, tail, and ventral surface face the cathode. 

 If placed at right angles to the current, the head turns to face the 

 cathode. The animals did not usually travel in the current ; the 

 margins of the body were kept in constant undulating movements ; 

 the animals were obviously much more stimulated when facing the 

 anode than when facing the cathode. 



VIII. ANNELIDS. 



i. The Metabolic Gradient. The gradient of the chaetopod 

 annelids has been described by Hyman ('16) and Hyman and 

 Galigher ('21). The gradient is of the double type, the metabolic 

 rate being high at anterior and posterior ends and decreasing from 

 both ends to the middle region. Tests of the oxygen consumption 

 of pieces from different levels show that posterior pieces consume 



