MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 553 



of the film. The interior end is unable to get any firm 

 attachment so that the rest of the body may be drawn 

 forward. Furthermore, simihir resistance is offered to the 

 stretching" of the head forward as to the turning of it 

 towards one side. 



When the animal is gliding on the surface film the same 

 raising of the head (with reference to the worm, of course: 

 in this case, a lowering with reference to the centre of the 

 earth) and waving it about in the water occurs as under 

 normal circumstances. In some cases two thirds or three 

 fourths of the whole body will be thus raised and waved 

 about, extending itself to its utmost capacity, and apparently 

 seeking some solid body on which to attach itselP. In these 

 cases only a small portion of the very posterior end of the 

 body will be left in contact with the film to support the 

 whole. 



On coming to the side of the dish when gliding on the 

 surface film, the worm almost invariably leaves the film and 

 turns down the side of the dish. The reaction which is the 

 cause of this and of the organisms passing from the side of 

 the dish on to the film will be brought out later. 



d. Relation of Movements of Triclads to those of 

 other Forms. — In respect to their movements, the triclads 

 studied occupy a somewhat intermediate position between 

 certain other groups. The rhabdocceles are in general 

 characterised by free movements in the water, brought 

 about by cilia covering the whole body. Their movement 

 in general features resembles that of the holotrichous 

 Infusoria. A type showing well this class of movement 

 among the Turbellaria is Stenostoma leu cops. The 

 movement is not at all or very little dependent upon muscular 

 activity. On the other hand, the movement of many of the 

 polyclads is characteristically muscular. An example of this 

 is found in the case of Leptoplana tremellaris, where 

 the movement is largely effected by the rhythmical beating 

 of the margins (cf. Lang, '84). In fact, tliis form of 

 movement has become so well developed in these animals 



