LOCOMOTION OF BERMUDIAN MOLLUSKS 229 



produces the same effect as if there were single longitudinal 

 fibers extending the whole length of the foot, each of which 

 contracts in turn. 



A second method of forward locomotion was observed in Cy- 

 praea. Waves often started from the border of the foot between 

 the most posterior point and a point on one side some three 

 centimeters anterior to it, and passed across the foot preserving 

 this same alignment. This therefore made waves which, as 

 they progressed, extended diagonally across the foot, disappearing 

 first anteriorly. In one case while the animal was going forward, 

 such diagonal waves which had been moving from left to right 

 suddenly stopped, and diagonal waves started up in the opposite 

 direction, i.e., from right to left, but without causing cessation 

 of locomotion or a change in the direction of the animal's move- 

 ment. When one watched a particular spot on the foot over 

 which diagonal waves were passing, it, was evident that the spot 

 was carried both forward and sidewise. In this case one must 

 suppose that another set of muscles running transversely in the 

 foot is also in operation, and that both longitudinal and transverse 

 muscles act simultaneously, producing locomotion in a direction 

 demanded by the law of parallelogram of forces. It would 

 seem therefore that the whole animal, instead of moving straight 

 forward, was probably moving forward and to one side at the 

 same time. The distances traveled by the animal were so short 

 (1 to 2 cm. at a time) that I was unable to determine this ac- 

 curately. If this supposition is true, a periodic reversal of direc- 

 tion of diagonal waves would keep the animal in an essentially 

 straight path. 



Forward locomotion, however, was seldom observed in Cypraea. 

 Usually the animal kept turning and moving in circles, now to 

 the right, now to the left; if to the left, waves started in the 

 anterior third of the foot about one-half- centimeter in from the 

 right edge, and extended from the extreme anterior end backward 

 about one-third the length of the foot. These (lateral) waves 

 passed sidewise from right to left. At the same time lateral 

 waves started on the left. side, extending from the center of that 

 side to the posterior end. Not only could there be two sets of 



