211] TURBELLARIA FROM THE MISSISSIFFI BASIN— HIGLEY 17 



their lives hidden away that they escape notice even tho they are often present. 

 Such is probably the case with many of the creeping rhabdocoels. 



Reactions of Worms 



This study of biologic conditions is at the outset superficial, and only the 

 most general conclusions can be ventured. Definite and positive statements 

 are to be questioned since so vast a number of factors is concerned, and it is 

 now an impossibility to know or control nearly all of them. Consequently, 

 many observations are qualified and results are guardedly and hesitatingly 

 set forth. But in studying any pond or stream, there are always many details 

 of hfe habit which present themselves conspicuously, and many of the influen- 

 ces which bring about certain effects can be very clearly recognized. 



Perhaps one of the most noticeable reactions is that to the amount of 

 oxygen. For altho supplied with no tissue or organ by which to make direct 

 use of oxygen or by which to effect an exchange of carbon dioxide, the Tur- 

 beUaria are all quite sensitive to the presence or absence of either. The pro- 

 cesses of respiration are carried on thru the skin, the parenchyma, and even 

 thru the general cells of the body itself which are in direct communication with 

 the surrounding water. Thus, altho the amount of oxygen necessary is in- 

 finitesimal, there is no reserve store, so that there must be a very constant 

 source. In aquaria, when there begins to be only a slight overloading with 

 carbon dioxide, many individuals can be seen making their way to the top 

 where there is a layer of fresh water. Oftentimes this seems to be the only 

 method of dragging certain species out from their hiding places. They are 

 hkely to remain near the top for days, staying quietly near the edge or gliding 

 about very slowly just beneath the surface. Sometimes around the edge of 

 the dish or the tank they crowd up into the httle film of water held by 

 capillarity above the general surface. In balanced aquaria, on the other hand, 

 large num.bers are often hard to find, since even the deeper portions will be 

 perfectly fresh. If, then, for any reason the balance is suddenly lost, up to 

 the top come several species. Some forms are habitually free swimming 

 and are much of the time to be found creeping over the surface or exploring 

 thoroly the deeper regions. When oxygen is lacking they remain con- 

 stantly in the uppermost water, avoiding the foul depths. Under normal con- 

 ditions many species find their most suitable habitat entangled in a mass of 

 alga or creeping over bits of weed, probably as much for the supply of oxygen 

 and consequent purer water as for a hiding place. This last conclusion is 

 evident since in those pools where there is Httle plant life, all the specimens 

 present are found in close proximity to whatever bits of green algae can be 

 found in the same way that any animal clings to its last source of oxygen. 



Planarians which live in locations vastly different from the above seem 

 just as sensitive, for even tho their habitat is running water, not all parts of 

 a stream are equally favorable. For example, the pebbles and rocks in shel- 

 tered, quiet pools show no specimens even when the food is ample. If, 



