J. M. D. OLMSTED 61 



Of the fourth type of movement Child makes the following 

 remark, ''The animals appear almost as if walking forward." 

 This ditaxic retrograde locomotion is brought about by the re- 

 leasing of a small portion of the lateral margin at the anterior end 

 of the worm, the pulling of this bit forward 2 or 3 mm. by con- 

 traction of the longitudinal muscles, and its reattachment at a 

 point anterior to its former position. The contraction once 

 started proceeds in a wave down the entire margin to the posterior 

 end. These waves alternate on each side of the body, so that the 

 worm appears to stride along like a biped. This agrees in every 

 respect with the process in gastropods. From records of observa- 

 tions on one large specimen of P. californica, the rate of slow 

 ditaxic locomotion is 0.21 cm. per second, and the worm takes 

 0.33 step per second. Other records on the same individual gave 

 0.36 cm. and 0.4 step per second. When disturbed it could pro- 

 ceed at the rate of 0.39 cm. per second in 0.66 step. The rate of 

 locomotion may therefore be varied both by an increase in the 

 number of steps and the length of each step as well. For P. 

 littoricola the ordinary movement was more rapid, i.e., 0.4 cm. 

 per second in 0.11 step. When disturbed, there was very little 

 increase — 0.43 cm. in 0.13 step. This type of locomotion was 

 not observed in E. cavicola. 



To determine whether the wave was initiated in the margin 

 of the head, this portion was cut away. It was then found that 

 waves started just posterior to the cut, but their rate was prac- 

 tically normal. In another experiment a semicircular cut in 

 front of the brain of L. saxicola severed the connection of the 

 nerves of the anterior margin with the brain. This worm ex- 

 hibited the w^alking movement perfectly, the waves starting on a 

 level with the ends of the cut. The cut healed over in three days, 

 and by the fourth day locomotion was perfectly normal, the 

 waves starting at the anterolateral margin of the head. Headless 

 specimens and those with the cephaUc ganglia removed were 

 unable to employ this type of locomotion, but anterior pieces 

 from worms transected just behind the brain nearly always use 

 this as their sole means of locomotion. 



THE JOtTRNAL OF EXPEHIMENTAL ZO JLOOY, VOL. 36, NO. 1 



