6 LEON J. COLE 



III. METHOD OF LOCOMOTION 



Before considering in detail the direction in which the star- 

 fish crawls it may be well to consider briefly the method of loco- 

 motion. In the paper to which reference has already frequently 

 been made Jennings ('07) does much to correct the popular 

 conception of this process. The important point is that except 

 on vertical or overhanging surfaces, the tube foot of the star- 

 fish acts essentially as does the leg of a higher animal, that is, 

 ''as a lever for swinging or shoving the body forward, not as a 

 rope for hauling it forward." The sucker functions chiefly in 

 giving the foot a firm hold and preventing its slipping. As a 

 result of this use of the tube feet, the locomotion is not a per- 

 fectly even glide, but may often be seen to consist rather of a 

 series of very short lunges. Furthermore, these lunges are not 

 always exactly in the same direction but now a little to this side, 

 now a trifle to that, resulting in a slight zig-zag movement, so 

 slight, however, that it is apt to be overlooked without close 

 observation. 



Different starfishes show considerable individuality in their 

 methods of progression. Certain specimens show a tendency 

 as they crawl, even over a short course, to change the direction 

 of locomotion, not by a turning of the body, but by a change of 

 front, so to speak. In an indisturbed individual this change 

 is usually very small; for example, if it starts out with ray e 

 in advance, it may change more or less to the interradial area 

 ea, but the arm e points in the same direction as before.^ In 

 the experimental trials, this change of direction was usually so 

 slight that it was inconsequential; when it was greater, the pre- 

 ponderating direction was recorded. In only four instances in 

 the course of these experiments was an undisturbed crawling 

 starfish, so long as it remained on the bottom of the dish, observed 

 to change its course radically, or to stop and then go off in a 

 new direction. In these four cases (specimen No. 3, trials 38 

 and 47; specimen No. 6, trials 12 and 16) there was no obvious 



* In some cases there may be a slight turning of the animal as well as this 

 change of direction. 



