554 RAYMOND PEARL. 



tliat wo may tliink of the margins of the body as special 

 locomotor organs. Cihary action phiys little if any part in 

 the movement of such a form. It is to bo noted, however, 

 that both the rhabdocoBles and the polyclads are capable of 

 performing' true swimming movements, i.e. movements free 

 in the water without contact with any solid body. In the 

 fi'osh-water triclads, especially of the genera Plan a via 

 and Doudrocoelum, the cilia have become much diminished 

 in comjiarison 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 throngh 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 high 

 development of muscular locomotion which the polyclads 

 have. There is a purely muscnlar 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 triclads seem to form a 

 transitional stnge 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 occupy a position very similar to that 

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



' 1 do not. wisli to imply, in lliis discussion of tlie did'crciit forms of move- 

 ment us rclaled to the number and distribution of the cilia, any belief that 

 structure gave rise to finiction or function to structure. I wisli merely to 

 jjoiut out, the evident correlation vvliieli exists in the matter. It seems to me 

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

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

 attem])ts to settle the phylogenetic develi>pnient of the ptienomena would 

 aiiiiear to he fruilless. 



