MOVEMENTS, KTCI., OF FRESII-WATER PLANARIANS. 545 



tion, represented disproporfcioiiately exaggerated iu tliick- 

 ness. This sticks to the surface of the substrate, and the 

 backward beating of the cilia drives tlio worm ahead. 



]. Rate of Gliding Movement. — There is no very 

 marked difference in the rate of the gliding movement in 

 case of the species of l^lanaria studied. On the whole, 

 specimens of P. dorotocephala move more rajjidly than do 

 those of the other two species, but there arc large individual 

 differences in this matter. Active specimens of Dendro- 

 coelum, sp., move much faster than any other planarians I 

 have observed. Large specimens of this form will sometimes 

 glide along with simply amazing rapidity, not showing the 

 slightest tremor of the surface of the body. 



As to the absolute rate of the crawling, some statistics 



yi^jy///f//Y///r7T//Tf/rr/r>J!:r//7Tt/^//-///:^/^7/,: 



Fig. 4. — Diagram Iu show \\\v. niecliauism of t.lic gliding movenient. A 

 represents a specimen of I'laiiaria seen from t,lic side ; W, the layer 

 of nniciis secreted by tiie animal. (This layer is represented as 

 greatly exaggerated iu lliickness in |)roportioii lo the animal.) C, 

 cilia. I), the subslrafe. The arrow indicates tJic direction in which 

 the organism is moving. For further explanation see text. 



have been collected and will be presented. The statistics 

 were obtained in the following way : — A paper was ruled into 

 centimetre squares; over this was placed a Hat Petri dish 

 containing the Avorm to be tested. Normal active specimens 

 of P. maculata were used, and nothing was put into the 

 dish but fresh clean water. The experiments were performed 

 at night, and the source of illumination Avas a 16-candle 

 power electric light enclosed within a ground glass globe. 

 This lamp was 35 cm. above and 35 cm. distant iu a hori- 

 zontal direction from the centre of the dish, so that the light 

 struck the animal at an angle of approximately 45° on its 

 dorsal surface. The worm was allowed to get into an even. 



