MOVEMENTS, ETC., OF FRESn-WATER I'LANAIilANS. 531 



plauarians. The " Bruckenfadcn," wliicli are formed by the 

 hiiid plauarians when they pass from one solid body to 

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

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

 under proper conditions they would not be formed. Lehnert 

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

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

 also noted that Poly cells tenuis was able to crawl back 

 upon a mucus thread after passing for some distance down 

 over it. 



Nothing- like the formation of "cysts" froui this mucus, \ P^'^ 

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

 Stichostemma, has been observed in the case of planarians. 

 Its only biological significance in these forms is in relation 

 to movetuent, 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 

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

 dislodge 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. Woodworth ('97) has 

 described a permanent anterior adhesive disc in Dendro- 

 coelum 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 glauds 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 margin or the posterior end will adhere so firmly to the 

 glass that the points of attachment are drawn out into 



