irOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 545 



tioiij represented disproportionately exag-gerated in thick- 

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

 backward beating of the cilia drives the worm ahead. 



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

 marked difference in the rate of the gliding movement in 

 case of the species of Planaria studied. On the whole, 

 specimens of P. dorotocephala move more rapidly than do 

 those of the other two species, but there are 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 



-*- 



<^^^^^/dr^/^>?!?^^ 



Fig. 4. — Diagram to sliow ilu; mecliaiiisni of the gliding movement.. A 

 represents a specitnen of Planaria seen fromUie side; B, the layer 

 of mucus secreted by tjie aniuKil. (Tliis layer is represented" as 

 greatly exaggerated in thickness in proportion to the animal.) C, 

 cilia. I), tiie substrate. The arrow indicates the 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 flat Petri dish 

 containing the worm 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-caudle 

 power electric light enclosed Avithin a ground glass globe. 

 This lamp vvas 35 cm. above and 35 cm. distant in 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, 



