MOVEMENTS, ETC., OF FEESn-WATKK PLANAKTANS. G99 



mass of detail. It is desirable, then, to state clearly at the 

 end the impoi'tant general facts which have been bronoht ont 

 by this study, and to discuss to some extent their significance. 

 In this place I shall state the results in a categorical manner, 

 making no attempt to indicate the evidence on which the 

 conclusions are based. This will avoid needless repetition. 



1. The locomotor movements of PI an aria are of two sorts, 

 gliding and crawling. The gliding movement is produced 

 by the beating of the cilia on the ventral surface of the 

 organism. It is by far the most usual method of locomotion. 

 For its production it is necessary that thei'e be a layer of 

 sticky, mucous slime between the ventral surface of the body 

 and the substrate. In this slimy secretion the cilia beat and 

 so propel the animal (cf. pp. 544 and 545). The organism 

 never moves freely through the water without some sort of 

 mechanical support. The rate of the gliding is changed by 

 the action of various agents, such as light, chemicals, elec- 

 tricity, etc. Its direction is always forward. 



The crawling movement is produced by strong longitudinal 

 waves of muscular contraction passing over the body from 

 the anterior to the posterior end. It is more rapid in rate 

 than the gliding. It appears only after strong stimulation 

 of the organism, and its purpose is evidently to get the 

 animal quickly away from harmful stinuili. Its direction 

 may be either forward or backward. 



Periods of movement alternate with periods of rest in the 

 course of the animaPs daily activity. When at rest the flat- 

 worm is in a condition of relaxation and generally lowered 

 tonus, corresponding to the condition of a higher organism 

 in sleep. The causes which induce the coming to rest are — 

 (a) a more or less fatigued condition of the organism. This 

 is the primary cause ; without it the other causes are ineffec- 

 tive. {!)) A relatively low intensity of light, (c) Koughness of 

 tlie substrate. This brings the body into a position such that 

 its different parts form angles with one another, and causes 

 the animal to couu' to rest as the result of a reaction wiiich 

 I have called goniutaxis (p. 5G2). (d) Certain chemical con- 



