64 LOCOMOTION OF POLYCLADS 



posterior end of the worm. The impulse therefore was carried 

 on through the nerve cord, though there was no visible effect 

 accompanying its passage. But if the wound were near enough 

 to the midline to have cut into the nerve cord, the wave on that 

 side ceased entirely at the level of the wound. According to 

 the diagram of the nervous system of L. saxicola of Heath and 

 ]\IcGregor, there is only one cross connective between the two 

 nerve cords, and that is situated immediately posterior to the 

 brain. If the wound is made anterior to this connective an oc- 

 casional wave will pass down the injured side as explained above, 

 but if the wound is posterior to this connective there can be no 

 conduction of the impulse to form a muscular wave. Local mus- 

 cular contraction can still take place, however. In one case a 

 long strip was cut from one side of the body of a worm, leaving a 

 narrow bridge of tissue at the anterior end to attach it to the 

 body. The ditaxic waves stopped when they reached the bridge 

 of tissue, and reappeared at the posterior edge of the wound. No 

 wave appeared in the strip itself. One could cause the strip to 

 contract by pricking any portion of it, so that the local muscular 

 response remained. Within two days the strip was attached to 

 the body except at the extreme posterior end, but no waves ap- 

 peared in it when the worm was 'walking.' On the third day the 

 wound was entirely healed and waves passed two-thirds of the 

 way down the strip. On the following day the animal moved 

 normally. The union of cut ends of the nerve cord or of severed 

 peripheral branches with the brain or nerve cords occurred in a 

 remarkably short time, since perfect coordination was established 

 in a very few days. But if a piece of the nerve cord were actually 

 removed, there was lack of coordination for several weeks, be- 

 cause of the relatively slow regeneration of lateral parts including 

 the nerve cords. Nerve cords do regenerate, while the brain 

 will not. 



The locomotion of these polyclad worms is comparable in every 

 respect with that of gastropod mollusks. The types of move- 

 ment are the same. Retrograde monotaxic, retrograde alternate 

 ditaxic, and ataxic methods are clearly distinguishable. The 

 part played by the nervous system is similar in each group 



