530 RAYMOND PEARL. 



The mucus also undoubtedly plays au important part in 

 the attachment of the animal to the under side oE the surface 

 film. When the worm leaves the surface film in open water, 

 i. e. when it cannot reach any solid body, it hangs by the 

 mucus thread in much the same way that a terrestrial 

 mollusc, like the common slug, does when it passes through 

 the air from a higher to a lower point. This observation I 

 have made many times, though generally in an indirect way. 

 As has been said before, the mucus thread is invisible when 

 first secreted, so that when a worm leaves the surface film it 

 seems to glide freely through the water. If, however, one 

 passes a needle horizontally through the Avater immediately 

 above the posterior end of a worm which has just left the 

 surface film, it will be seen that at a certain point (where the 

 needle strikes the thread) the end of the worm will be jerked 

 to one side. Furthermore, one may with care pick up the 

 invisible mucus thread with forceps and raise the whole 

 worm, provided the attempt is made before the anterior end 

 reaches the bottom. I have seen specimens of P. maculata 

 craAvl back upon the thread after going a part of the way 

 down to the bottom, and again regain a position on the 

 surface film. The same thing is frequently done by slugs. 

 When the animal has fully reached the bottom, connection 

 with the thread which has served to suspend it in the water 

 is usually broken by several sharp jerks of the posterior end 

 of the body from side to side. 



The relation of the organism to this slimy secretion is 

 much the same in the land plauarians, according to the 

 observations of Lehnert ('91). He distinguishes in case of 

 these forms " Kriechfaden,^' " Briickenfaden,'' and " Gleiten- 

 faden " formed fi'om the slime, the distinction being based 

 on the relation of the thread to the surroundings. The 

 " Kriechfaden " are the threads left behind as the organism 

 moves over a continuous solid body, and the " Gleitenfaden " 

 are the threads on which the animal hangs in passing 

 throuo-h the air from a hio-her to a lower level. Both these 

 forms of threads I have found in case of the fresh-water 



