215] TURBELLARIA FROM THE MISSISSIPPI BASIN— HIGLEY 21 



sources of disintegrating matter completely used up, or masses of debris may 

 be washed away to such an extent that a whole area is entirely cleaned of its 

 food content. Under such conditions the turbellarian does not die imme- 

 diately. It goes thru a process of starvation so slow as to be almost unno- 

 ticeable. The individuals become thinner and smaller and shrink gradually 

 until in some specimens they are no more than one-fifteenth their original size, 

 at the same time becoming almost transparent in color. This explains the 

 immense amount of variation in size of adult forms, the individual dimensions 

 depending directly upon the food taken. 



The enemies of the turbellarians are few. Altho such delicate animals are 

 evidently almost entirely defenseless, they manage to carry on a fairly free and 

 unhampered existence. The smaller species spend much of their time swim- 

 ming slowly in open water where they encounter almost all the other inhabi- 

 tants. It is especially noticeable in aquaria where many species collect at 

 the surface or towards the light that the rhabdocoels or planarians mingle 

 constantly with the others, crawling over them and around them in the most 

 unconcerned manner. And often two or three individuals will collide with 

 each other without the slightest inconvenience or apparent fear. This is 

 easily explained in the case of the planarians which are enormous compared 

 with the tiny crustaceans, but the rhabdocoels are so nearly the size of the 

 other cormnon types that they seem possible prey. However, a number of 

 times rhabdocoels have been found gliding unharmed directly thru between 

 the valves of an ostracod or resting contentedly under the edge of the carapace 

 of a cladoceron. 



This situation may be due in part to the fact that the crustaceans them- 

 selves habitually Hve upon disintegrating material and are not likely to attack 

 living forms. Then, too, some of the rhabdocoel species possess nematocysts 

 which would make them not only unfit for food but also rather well protected 

 from most enemies. Others which do not possess stinging cells have especially 

 developed dermal rhabdites in very great numbers. While these are not 

 weapons of defense, they probably render the individuals unpalatable. 



Perhaps the most effective method of protection is the general habit of re- 

 tiring to well-guarded situations. A flat worm lying close on the underside 

 of some rough stone is in a fairly safe place, where few hostile species are capable 

 of dislodging it. The rhabdocoel hunts in some mass of alga where, at the 

 same time, it is completely hidden from many large forms. If, however, it 

 encounters an enemy of any sort, the rapidity with which it can contract 

 enables it to disappear. 



