334 ASYMMETRY IN CERTAIN LOWER ORGANISMS, 



forward course and when stimulated to change its course. The arrangement of the 

 toes is merely another adaptation to this. If the toes were so arranged as to bend 

 downward, a sudden stroke with them would turn the organism toward the ventral 

 side, quite in opposition to the other tendencies of the animal. But with the toes 

 turning to the right, their action is brought into harmony with the rest of the 

 behavior of the organism. On getting to a place where it can go forward no farther 

 or upon other strong stimulation, the animal turns its toe or toes suddenly and 

 strongly to the right and forward. By this the usual turning to the right is strongly 

 accentuated; the path of the animal is suddenly changed. 



The degeneration of the right toe and increase in size of the left can hardly be 

 considered to follow directly from the factors thus far adduced. This change, per- 

 haps, becomes intelligible, however, as an adaptation to another habit of the animals 

 not yet considered. The Rattulidse frequently use the toe or toes as an axis of rota- 

 tion while maintaining a fixed position, in the following manner. There are two 

 mucous glands in the posterior part of the body (Fig. G, 10, gl.muc.) which secrete a 

 tenacious fluid that is stored up in two large mucous reservoirs (rsv.muc.). From 

 these the secretion passes out between the base of the toe and the substyles, being 

 directed by the latter down along the surface of the toe. From the tip of the toe 

 it trails off into the water, like a spider's web, and attaches itself to any object with 

 which it comes in contact. The animal then remains suspended in the water like a 

 spider from its thread. It spins about on its long axis, remaining meanwhile nearly 

 in the same position, or it may of course move in the circumference of a circle about 

 the object to which it is attached. While thus attached, the action of the cilia brings 

 food to the mouth, just as in the case of the Rotifera that are permanently fixed by 

 their posterior ends. The free-swimming rotifiers which have this mucus secretion 

 have the advantage, therefore, of being able to change temporarily their roving 

 method of life into a fixed one. 



This habit is one of the most characteristic features in the behavior of the Rat- 

 tulidse, and at such times, as stated above, the animal rotates about its point of 

 attachment as on a pivot. Now it is very evident that a single long toe would 

 serve much better as such an axis of rotation than would two short toes side b> side. 

 In the latter case much resistance to the rotation would be caused. 



It is probably, then, as an adaptation to this habit that the right toe has degen- 

 erated, while the left toe has increased in size. Favorable conditions for such a 

 change have been established by the unsymmetrical position of the two toes, already 

 described. Since they no longer lie side by side, the two toes no longer have the 

 same relation to the environment as they have in a bilaterally symmetrical animal. 



