70 ILLINOIS BIOLOGICAL MONOGRAPHS [264 



will always find their way to the hghted part after some time has elapsed. 

 This was also true in large aquaria, but the response was not as constant, due 

 to influence of other factors. 



The feeding habits seem to be the result of the surrounding conditions. 

 Altho active and much of the time living in open water, these animals do 

 not attack living forms. Live, hard-shelled Cladocera'and rotifers would 

 be difficult for such soft, unarmed forms to manage, and even the smallest 

 protozoa appear to be safe from their depredations. Repeatedly, upon en- 

 countering a most defenseless protozoan, the rhabdocoel will glide over it and 

 leave it imharmed. However, the whole attitude is changed when the prey 

 is dead. Very often the dead shells of small Crustacea, some of them many 

 times the size of the rhabdocoels, are found entirely covered by an active 

 hungry mass, eagerly devouring every bit of edible portion. It is quite im- 

 possible to dislodge even a single individual without crushing or injuring it, 

 so tenacious is their hold. Figure 3 shows an empty shell of a Cyclops being 

 cleaned out thoroly by a few individuals. As they worked, it was possible to 

 see how industrious and intense was the habit and manner of finding every 

 particle of edible material. They remained within the shell for more than an 

 hour, constantly at work, until it was completely emptied of all soft material 

 and ready to fall apart. The animals themselves were by that time fully 

 gorged and showed the digestive cavity dark and large, obscuring even the 

 pharynx rosette. Ostracod shells seem easy of access and are very often cleaned 

 out In this same manner. Dead protozoa, also other soft bits of animal debris, 

 are eagerly attacked and devoured and it is probably in search of such particles, 

 which may be caught in the masses of algae or have reached the upper layer 

 of soft bottom mud, that the individuals are seen in large numbers finding their 

 way hither and thither in the deeper portions. This may also explain the 

 fact that water, dark with masses of organic debris, forms good feeding ground, 

 so that instead of being an unendurable condition, it is one in which there is 

 possible greater vegetative activity. The role of scavenger is thus played 

 by these small forms and their place in the balance of animal life must be an 

 important one. Hardly to be considered as enemies of even a single individual, 

 they are merely one of the types without which other more aggressive species 

 could not exist. 



It seems probably that so unarmed and defenseless a creature as this must 

 have a number of enemies, 'but evidently they are almost negligible. From 

 an enemy like the fish, which takes in large amounts of water, the tiny rhab- 

 docoel has no escape and no method of defense, and numbers must thus be- 

 come the food of large forms. Others will stray into the jaws of some animal 

 along with the water, e.g., the crayfish, but the smaller Crustacea seem to 

 make little headway in an attempt to kill and eat these dainty morsels. Ostra- 

 cods have a habit of attacking a resting individual and by means of strong 

 appendages tearing a hole in the skin. It takes a good deal of persistence to 



