554 RAYMOND PEARL. 



that we may think of tlie margins of the body as special' 

 locomotor organs. Ciliary action plays little if any part in 

 the movement of sucli a form. It is to be noted, however, 

 that both the rhabdocoeles and the polyclads are capable of 

 performing true swimming movements, i.e. movements free 

 in the Avater withont contact witli any solid body. In the 

 fresh-water triclads, especially of the genera Plannria 

 and Dendrocoelum, the cilia have become much diminished 

 in comparison with the rhabdocoeles, and are restricted to a 

 portion of the ventral surface only. Consequently they are 

 not numerous and strong enough to support and move the 

 disproportionately heavier body freely through the water. 

 The movement of the cilia merely serves in these forms to 

 propel the body while insufficient to support its weight. 

 Consequently we find the principal form of movement to be 

 a gliding over the surfaces of solid bodies.^ On the. other 

 hand, the fresh-water triclads have not attained the liigh 

 development of muscular locomotion which the polyclads 

 have. There is a purely muscular movement in their case, 

 but it is not by far the most important form of locomotion, 

 and is not so highly developed as is that of the polyclads. 

 Evidently, then, the fresh-water ti-iclads seem to form a 

 transitional stage in respect to locomotor phenomena between 

 the rhabdocoeles on the one hand, where purely ciliary 

 locomotion obtains, and the polyclads on the other hand, 

 where we find the locomotion largely if not entirely 

 muscular. Whether this has any phylogenetic significance 

 is not certain. 



The land planarians occup)^ a position very similar to that 

 of the fresh-water forms so far as their movements are 



' I do not wisli to imply, in tills discussion of tlie different forms of move- 

 ment as related to the number and distribution of tiie cilia, any belief that 

 structure gave rise to function or function to structure. I wish merely to 

 point out the evident correlation wiiich exists in the matter. It seems to me 

 most probable that structure and function changed together ; but in this, as 

 in many otlier similar cases, positive evidence is lacking, and consequently 

 attempts to settle the phylogenetic development of the plienomena would 

 appear to be fruitless. 



