228 J. M. D. OLMSTED 



able to proceed. As a rule the waves moved laterally, i.e., from 

 side to side, instead of from anterior to posterior or the reverse. 

 It was very strange to observe the whole animal moving forward 

 while the waves which caused this movement were actually 

 going sidewise. A single wave presented the appearance of a 

 deep brown streak about 4 mm. wide, bounded anteriorly by a 

 narrow (0.5 mm.) band lighter in color. When the animal 

 was moving forward, waves made their appearance at the pos- 

 terior edge of the foot, starting like ordinary direct monotaxic 

 waves. But after each one had moved forward 0.5 to 1 cm., 

 it suddenly bent forward, lengthened, and formed a wave which 

 extended the full length of the foot at about 1.5 cm. from either 

 the right or left side. If the wave extended along the left side 

 of the foot, it then travelled to the right, keeping its front parallel 

 to the long axis of the body with the narrow lighter band in 

 advance. In the majority of cases the waves went from left 

 to right, though in several instances the reverse was true. I 

 have called such wave motion lateral, though the resulting 

 locomotion is forward. 



One could readily observe the movement of a single point on 

 the foot, since the fine lines present could be used as landmarks. 

 It was seen that as a wave passed over any particular point, it 

 was carried forward, though the wave itself was moving from 

 left to right, or right to left. Parker's ('11) scheme for gastropod 

 locomotion can be applied in this case. The pedal wave is 

 considered as the result of two different muscular contractions; 

 first, contractions of "the dorsoventral muscles lift the foot 

 locally from the substrate," and, secondly, "the contraction of 

 the longitudinal muscles" causes "forward movement of that 

 portion of the foot which is temporarily lifted from the substrate" 

 and "extends the relaxing posterior fibers." But instead of 

 these contractions taking place "in sequence from behind for- 

 ward," as in Chiton tuberculatus which exhibits retrograde 

 locomotion in Cypraea during lateral wave motion they must 

 occur practically simultaneously along the whole length of the 

 foot, the series of fibers next to contract being those at one side 

 of the contracted ones, instead of those anterior to them. This 



