Pearl, Reactions of Limidus. 1 5 i 



strokes is certain, from evidence to be presented later. To such 

 an extent is the organism unable to direct its movement in 

 these early stages that, instead of tending to swim up from the 

 bottom, it actually tends at times to swim downward. In many 

 cases I have seen the beating of the gills become so forcible as 

 to raise the abdomen and send the embryo skimming along the 

 bottom on the antero-haemal surface of the cephalothorax. If, 

 under these circumstances, the anterior margin of the cephalo- 

 thorax happens to meet an obstacle in its path the embryo will 

 in many cases turn completely over on its anterior end as a 

 pivot, and come down with the haemal surface uppermost. 

 The "summersault" in such cases is caused solely by the con- 

 tinued violent swimming movement of the gills while the an- 

 terior end is held. 



How then does the embryo get up from the bottom at this 

 stage, so as to swim freely through the water ? This is done 

 in one of two ways, during the earliest stages after hatching. 

 The first of these methods is purely accidental so far as the or- 

 ganism is concerned. If an embryo which is sliding along the 

 bottom as the result of the violent swimming movement of the 

 gills happens to strike squarely a very small obstruction in its 

 path, the anterior end of the body will in some cases slide up 

 onto the obstruction. This, of course, gives the body as a 

 whole an upward tilt and if the swimming movements continue 

 the embryo will rise clear of the bottom and swim freely through 

 the water. Sand grains and pieces of cast egg membranes usually 

 serve as the means for starting the animals upward in this way. 

 Rising in this manner only occurs infrequently, since it is not 

 often that all the necessary conditions will be fulfilled together. 

 After the embryo gets started in this way it is able to sustain 

 itself in the water for as long as a minute even in very early 

 stages of its free existence. Such cases show that the ordinary 

 inability of the embryo to rise from the bottom is not due to 

 lack of force in the swimming movements. 



The second and more usual method by which very young 

 embryos rise from the bottom is by first turning over from the 

 usual position with the haemal side down, and then starting to 



