1911] Watson: The Genus Gyrocotyle. 375 



2. " Righting-up " movements, in which the worm turns from 

 oiii' surface to the other. These take place when the worm is 

 placed on its dorsal surface and also when it is exposed to strong 

 light. 



'A. This turning over is accomplished in two typical ways, 

 either by turning completely over on one side on the frills as an 

 axis, or by a more elaborate process of tucking under the aceta- 

 bulum until it emerges posterior to the rosette, when the rosette 

 is elevated and the overturning accomplished after the manner of 

 a somersault. 



4. A rotation from side to side of the terminal rosette, accom- 

 panied by a wave-motion in the margin of the frills, takes place 

 during the worm's most active period. 



5. Locomotion in a definite direction is accomplished by means 

 of a series of expansions and contractions, with some help from 

 the mobile lateral frills. The acetabular extremity is in every 

 case directed anteriorly ; the rosette is held in the typical posi- 

 tion, raised, with the canal opening ventral and the funnel open- 

 ing dorsal. The acetabulum is thrust out, and the whole body 

 elongates, the rosette remaining stationary. This expansion 

 leaves the rosette end where it was before, the acetabular cud 

 some two centimeters further toward the goal than at start. 

 Then a wave of contraction sets in, bringing the rosette end 

 nearer the acetabular end, which also moves backward a short 

 distance, approaching the rosette. The net result is an advance 

 in the acetabular direction. The acetabulum remains contracted, 

 while the rest of the body expands. In this expansion the rosette 

 must either retreat, or the whole contracted anterior extremity 

 be shoved forward. The latter is what actually occurs, as shown 

 by careful marking of the relative positions of the two ends 

 before and after the wave of expansion. This method of pro- 

 cedure is very effective, resulting in an average gain of about 

 two centimeters for each wave of contraction. A distance of 

 fourteen centimeters was traveled in ten minutes by one speci- 

 men ; but in this case more than the average gain was made, the 

 whole distance being accomplished in five contractions. 



