230 J. M. D. OLMSTED 



waves moving in opposite directions, but in several cases three 

 separate sets were to be seen. Once, when the animal was 

 turning to the left, waves in the anterior third passed from right 

 to left, in the middle third a second set passed from right to 

 left, but at a different rate and not continuous with the anterior 

 waves, while in the posterior third of the foot, waves were moving 

 from left to right. At another time, when the animal was 

 turning to the right, waves in the anterior part of 'the foot passed 

 from left to right, in the middle from right to left, and in the 

 posterior from right to left. In every instance, however, the 

 waves in the anterior third were moving in the same direction 

 that the whole animal moved, and in the majority of instances 

 the waves in the posterior third passed in the opposite direction 

 (fig. 1). This would naturally accomplish a quicker turn than 

 any other combination, and would therefore be what one might 

 expect to find. Several instances were, however, noted in which 

 the posterior waves were moving in the same direction as the 

 anterior ones. 



A still further complication occurred when an antero-posterior 

 wave appeared at the same time with the lateral waves. This 

 retrograde wave was seen only when the animal was turning 

 rapidly. It occurred on the inside of the curve which the foot 

 made when bending around, i.e., if the animal was turning to the 

 left, the retrograde wave appeared on that, the shortened, side. 

 It was a much wider wave than any of the others, had no light 

 area preceding the brown band, and extended as a rule only half 

 the width of the foot. It did not seem to interfere with the 

 lateral waves, which moved perpendicularly to it. The two 

 waves passed over each other and neither was interrupted. One 

 could see the lateral wave anterior and posterior to the retrograde 

 wave, and the retrograde wave on each side of the lateral one. 

 To this type of locomotion I have given the name composite. 



Another strange condition was observed when the Cypraea 

 was held at such an angle with a glass plate that it could attach 

 only the very anterior end of the foot. Waves appeared spread- 

 ing out in a fan-shaped manner from the center of the foot, 

 in some cases half of them going to the right and half to the left, 



