Pearl, Reactions of Liniubis. 157 



unsteady walking movements the body gets pointed in a new 

 direction such that the obstacle no longer completely stops the 

 progressive motion. Then, of coure, the embryo is able to push 

 around, and continue on its way. A large number of observa- 

 tions were made on this subject to determine whether older 

 embryos developed any distinct purposeful reaction to enable 

 them to get around obstacles. I was unable to convince my- 

 self that there was any real progressive development in this 

 matter. Sometimes older embryos will stop walking when they 

 strike an obstacle, fall back perhaps half the length of the 

 body, and then start forward again in a path at an angle to the 

 former line of motion. This will take them by the obstacle at 

 once. This behavior, however, does not occur with sufficient 

 frequency to warrant considering it a typical reaction. The 

 typical behavior throughout the "trilobite" stage seems to be 

 essentially that first described. 



Burrowing Reaction. — If Limuhis embryos in the "trilo- 

 bite'' stage are taken out of water and put on moist sand they 

 will usually in a short time burrow into the sand so as to be 

 completely buried. This behavior is of a character which would 

 warrant it being characterized as instinctive. Analysis shows, 

 however, that it is capable of explanation in another way. 

 When an embryo is placed on the sand it starts walking in the 

 usual way. This continues until some obstacle in the path 

 stops further movement ahead. Then, as has been described 

 in the preceding section, the animal pushes against the obstacle. 

 Now when this action takes place on sand the sand grains are 

 pushed out from below the embryo as a result of the action of 

 the legs. The violent movements of the legs tend to raise the 

 abdomen, and the anterior end gets pointed more and more 

 down into the sand. As walking movements of the legs con- 

 tinue more and more sand is thrust from below the animal, and 

 the whole body is thrust downward and forward. In this way 

 the burrowing is brought about. Any sort of obstacle in the 

 path will induce the burrowing when the embryo is on sand, so 

 far as I have observed. A needle held in front of the animal 



