J. M. D. OLMSTED 65 



(cf. section of "Physiol()o;ie des Nervensystems," by S. Baglioni 

 in Winterstein's "Handbuch der vergleichenden Physiologie"). 

 Slugs are able to move by means of waves on the foot when the 

 connection with the brain is severed. Waves can also appear on 

 isolated pieces of the foot. This is correlated with the presence 

 of an extensive nerve net in the foot with many cross connectives. 

 Ataxic locomotion in the polyclads is likewise carried on in the 

 absence of the brain and in pieces from any part of the body. 

 Their nerve net must be responsible for this movement, since the 

 movement is under the control of the nervous system in the im- 

 mediate vicinity of the contracting muscles. In snails, such as 

 Helix pomatia, the impulse which causes the normal peristaltic 

 wave arises in the pedal ganglion, and is transmitted by the nerve 

 cords of the foot, each of which serves a definite area. In a simi- 

 lar way both monotaxic and ditaxic locomotion of the polyclads 

 are controlled by the cephalic ganglia, and the nerve cords trans- 

 mit the impulses to a definite area. This is another instance of 

 evolution along the same lines in two quite different groups. 



SUMMARY 



The polyclads, Planocera californica, Phylloplana littoricola, 

 and Leptoplana saxicola, exhibit four possible types of locomotion. 



For the swimming movement it is necessary that the cephalic 

 ganglia be intact. Injury to or loss of the brain prevents the 

 use of this method. 



Ciliary action is not under the control of the nervous system 

 and plays practically no part in locomotion. 



Ataxic locomotion is a purely local phenomenon, but controlled 

 by the nervous system since it is abolished by chloretone. Eury- 

 leptotes cavicola moves by this type of locomotion alone. 



Ditaxic retrograde locomotion is under the control of the 

 cephalic ganglia, each ganglion controlling the progression of mus- 

 cular waves on its own side. 



The nerve cords serve as conductors for the impulses to wave 

 formation each on its own side. Cutting a nerve causes the 

 waves to disappear on that side at the level of the cut. 



The locomotion of polyclads is comparable in these respects 

 with that of mollusks. 



