I 5 o Jonrnal of Comparative Neurology and Psychology. 



like the respiratory movements except that the ampHtude and 

 force of the beats are much greater in the swimming than in 

 the respiratory movements. On account of the fact that the 

 gills are extended so as to form nearly a right angle with the 

 body at the beginning of each beat a considerable portion of 

 the effective force of the stroke is directed nearly straight back- 

 ward. 



There is a very marked and fundamental difference be- 

 tween the swimming movements of the embryo and the adult 

 with respect to the thoracic appendages. In the adult Limuhis 

 all the walking legs beat strongly back and forth in time with 

 the gills during the swimming. As the gills are raised the legs 

 are extended and thrown far forward in the cephalothorax, and 

 as the gills strike backwards the legs accompany them in this 

 movement. In the embryo of the stage under discussion, how- 

 ever, the legs take no part whatever in the swimming move- 

 ment. Whenever the gills begin swimming motions the legs 

 are extended as much as possible and thrown forward until they 

 touch the antero-lateral margins of the cephalothorax. Then 

 they are held rigidly in this position as long as swimming con- 

 tinues. The appearance in swimming is as if the legs became 

 temporarily paralyzed and held in a "forced" position while the 

 gills were performing swimming movements. 



A very interesting course of development is to be observed 

 in connection with the ability of the embryo to direct its move- 

 ments when swimming. The usual position of the embryo, if 

 lying by itself in the dish, is the same as its position in the 

 "vicarious chorion," namely, lying on the bottom in an inverted 

 position. As has already been stated, swimming movements 

 begin at once after the embryo is hatched. For some time 

 after these movements begin, however, the embryo is not able 

 to rise from the bottom. The gills beat most energetically, but 

 the only result is to cause the embryo to slide along the smooth 

 bottom of the dish. It has not acquired the faculty of so bend- 

 ing the abdomen with reference to the cephalothorax as to 

 make the force of the swimming raise it from the bottom. 

 That this failure to rise is not due to lack of power in the gill 



