MOVEMENTS, ETC., OP FRESH-WATER FLANAKIANS. 531 



planariaiis. The '" Briickenfaden," wliicli are formed by the 

 hind plauariaiis when they pass from one solid body to 

 another at about the same level, I have never observed in 

 case of fresh-water planarians, though I see no reason why 

 under proper conditions they would not be formed. Lehnert 

 (loc. cit., p. 17) says, " Die Wasserplanarien bilden Avie die 

 Landformen ihren Kriech-, Briicken-, und Gleitfaden." He 

 also noted that Poly celis tenuis was able to crawl back 

 upon a mucus thread after passing for some distance down 

 over it. 



Nothing like the formation of '^ cysts" from this mucus, 

 such as Child (: 01, pp. 989 to 993), found in the case of 

 Stichostemnia, has been observed in the case of planarians. 

 Its only biological significance in these forms is in relation 

 to movement, as pointed out above. 



In connection with the subject of mucus secretion it may 

 be well to point out the tenacity of the attachment of the 

 flat-worm to the bottom. It will be found in attempting to 

 dislodo-e the animal that the extreme anterior end and the 

 extreme posterior end stick very firmly to the substrate. 

 Whether this holding is the result of a sucker-like action of 

 the ends of the body, or is due merely to the stickiness of the 

 mucus, I have been unable to decide. It is easily possible 

 that the muscles could be so contracted as to form out of 

 either end of the body a practical sucker, but whether this is 

 done or not it is impossible to say. Wood worth ('97) has 

 described a permanent anterior adhesive disc in Dondro- 

 coclum lacteum, but considers that this "is not a true 

 sucker, nor does the animal employ its anterior end for the 

 purpose of attachment to any greater degree than the 

 posterior or lateral margins of its body, along the ventral 

 surface of which numerous mucus glands have their 

 openings. In truth, it is the margins and posterior end that 

 adhere more firmly to a support ; often when the animal is 

 forcibly removed from the sides of the aquarium the parts of 

 the nnirgin or the posterior end will adhere so firmly to the 

 glass that the points of attachment are drawn out into 



