MOVEMENTS, ETC., OF FiiESH-WATEU PLANAUIANS. 529 



Avliicli is apparently quite generally distributed among lower 

 organisms^ may be an important source of error in work on 

 the behaviour. 



Regarding the statement of Bardeeu (: 01, p. 14) that he 

 does not find that Planaria " is sensitive to anything but light 

 and contact/' nothing need be said here. The detailed 

 accounts of the reactions of the organism to a variety of 

 stimuli which follow in this paper are in themselves a 

 sufficient criticism. 



h. Secretion of Mucus. — There is secreted at all times 

 over the surface of planarians a sticky slime, apparently of 

 the nature of mucus. This secretion is increased when the 

 animal is irritated, and is under normal conditions more 

 abundant on the ventral than on the dorsal side. If a 

 needle or fine glass rod is touched several times on the 

 surface of the body its end becomes covered with this 

 secretion. For this reason it is necessary in applying 

 localised mechanical stimuli to wipe the mucus off the end 

 of the needle frequently, in order to obtain good results. 

 Similarly, if one is using a sharp scalpel to cut the animals, if 

 the edge is left in contact with the surface of the body any 

 length of time before the decisive cut is made, the edge will 

 become so coated with mucus that a clean cut is impossible; 

 instead, the animal will slip from under the knife. 



When the animal moves about it leaves behind a more 

 or less heavy string of this mucus, so that if sevei'al speci- 

 mens ai'e placed in a clean glass dish the bottom will, in 

 a short time, become covered with a network of interlacing 

 mucus threads. The same phenomenon occurs in other 

 Turbellaria and among the Nemerteans (cf. Child, : Ul, 

 and Wilson, : 00). The threads when first secreted are so 

 transparent as to l)e invisible, but in larger quantities they 

 appear opalescent, and may be picked out of the dish with 

 forceps. 



The function of this secretion in locomotion is evidently to 

 attach the body to the substrate. Secretions for such a 

 purpose occur widely among aquatic organisms. 



