No. 5-] 



THE HABITS OF P 



199 



in these. The articulations are so arranged that there is little 

 chance for motion outside the vertical plane, the leg thus 

 moving up and down in the same plane in which it extends 

 from the body. There is, however, a possibility of movement 

 of the leg backward and forward in the horizontal plane to a 

 certain extent, this motion occurring chiefly in the articulation 

 between the first and second joints. The principal movement 

 of the leg, that is to say the movement in the vertical plane, 

 when the animal is swimming free, is shown approximately in 

 the diagram. Starting with the leg in the position shown at A, 

 and considering only that portion distal to the third joint, the 

 next movement is essentially a straightening out and raising 



,B 



J 



FIG. 2. Diagram showing movement of leg in Anoplodactylus. 



dorsally of that part, bringing it into a position as shown at B. 

 The leg is now extended still farther and brought downward to 

 C, then inward, and at the same time flexed, to the original 

 position, A. This serves to indicate the movements in a rough 

 way, and it can be readily seen that so long as the specimen 

 is free in the water and all the eight legs are working with this 

 same treading motion, the tendency is to propel the animal 

 dorsalward, that is, in a direction perpendicular to its dorso- 

 ventral plane ; the fact that the legs extend radially from the 

 body (Fig. i) would help to keep the animal going straight in 

 this direction, provided tliey all beat ivith equal force. If the 

 claw of .any leg should grasp a solid object as it comes down 

 from B to C, the movement CA would pull the animal in the 

 direction of that object. 



