138 MANTON COPELAND. 



the lateral margins of the foot, it does at times extend to this 

 region when, under favorable conditions for observation, it is per- 

 fectly clear that the wave is a band-like area of the foot which is 

 temporarily raised above the substrate and moved forward. This 

 interpretation of the pedal wave was offered by Parker in 1911, 

 and later (1917) was shown by him to be true in the case of 

 Aplysia. Olmsted ('17) also came to the same conclusion from 

 his studies of Bermudian gastropods. 



The speed of muscular locomotion was compared with that de- 

 pendent on ciliary action alone. A snail whose foot measured 

 about 10.5 cm. in length by 5.7 cm. in width was moving over the 

 bottom of a lead-lined tank in which the water was over 3 cm. 

 deep. By taking five records and averaging them, it was deter- 

 mined that five complete waves passed over the foot in 27.6 seconds, 

 during which time the snail moved forward 7.8 cm. Thus a wave 

 appeared on the average every 5.5 seconds and advanced the foot 

 approximately 1.5 cm. It was moving, therefore, at the rate of 

 one centimeter in about 3.5 seconds. A larger individual with a 

 foot about 15.5 cm. long and 10.7 cm. wide moved much faster. 

 When the results of five trials were averaged it was found that the 

 snail was travelling at the rate of 18.5 cm. in 31.6 seconds, or 

 stated as in the preceding case, a wave started every 6.3 seconds 

 and moved the foot ahead about 3.7 cm. Accordingly the rate of 

 locomotion was approximately a centimeter in 1.7 seconds. Com- 

 paring these locomotor rates with those determined for several 

 snails moving without rhythmic contractions, it is evident that the 

 muscular form of progression is faster. For example, the smaller, 

 more slowly moving individual of the two noted above was timed 

 as it glided by ciliary action over the glass bottom of a dish of sea 

 water, a favorable substrate for this type of movement. Twenty 

 records of its speed over distances of one centimeter were taken. 

 The fastest noted was 5 seconds, whereas the series averaged 5.7 

 seconds. 



Undoubtedly pedal cilia are beating when waves are passing 

 over the foot, for they unquestionably are when the snail is bur- 

 rowing into the sand and the same sort of contraction is in prog- 

 ress. The muscular form of locomotion may pass into the purely 

 ciliary one by the gradual reduction of rhythmic pedal movements 



