Pearl, Reactions of Limulus. ' 155 



an embryo has reached this position it will suddenly release the 

 legs from the cramped position into which they are thrown 

 when the gill swimming begins, and start scratching with the 

 chelae against the bottom in front of the anterior margin of the 

 cephalothorax. The evident purpose in this act is to help out 

 the gills at the critical moment, with the legs. The result usu- 

 ally is that the embryo immediately gets over into the normal 

 upright position. This reaction is of interest as indicating the 

 greater strength of the positive thigmotaxis of the chelae as 

 compared with the tendency for the legs to be held in a forced 

 position during the gill swimming. The tips of the chelae 

 when the legs are in the forced position during the swimming 

 project in front of the margin of the cephalothorax. When 

 the posterior end of the embryo is raised far enough the chelae 

 will evidently touch the bottom, and this stimulus is sufficiently 

 strong to overbalance the tendency for the legs to hold the 

 cramped position, and to cause them to kick against the bottom. 



The stimulus which calls forth all these attempts of the 

 embryo (the same is also true of the adult) to right itself, comes, 

 I believe, primarily, from the strong positive thigmotaxis of the 

 neural surface of the body, and especially of the margin of the 

 cephalothorax and the tips of the chelae, together with a nega- 

 tive thigmotaxis of the haemal surface. 



As the time of the moult which terminates the "trilobite" 

 stage approaches a very definite and curious reaction of the 

 thoracic appendages develops. In its typical form this reaction 

 is as follows : let a needle, or any other object of similar size, 

 be held parallel to the long axis of the body so that the ends 

 of one or more of the chelae touch it in the course of their 

 movements. Immediately all the legs will close over the needle 

 and, in a way, hug it up to the body. At once the embryo 

 begins to crawl along on the needle held in this way. Even if 

 the needle is held so as to be perpendicular to the bottom of 

 the dish the embryo will crawl up on it as far as the surface of 

 the water, and even in some cases rise partly out of the water. 

 So firm a grasp do the legs have on a smooth needle that with 

 moderate care an individual may be raised entirely out of the 



