FREE-LIVING FLATWORMS 



simple division of individuals to increase their numbers. 

 The common Planaria maculata divides transversely just 

 behind its pharynx (Fig. 94), only a little constriction show- 

 ing there at first. This grows deeper till the two parts hang 

 together by a mere thread, finally separating and moving 

 away from each other. The head soon grows a new tail, 



Fig. 94. — Two individuals of Planaria being 

 formed by automatic division of the body. 



and the tail region a new head. This kind of division is a 

 very frequent performance among planarians; one often 

 finds a partly divided planarian moving comfortably over 

 a leaf, no longer one animal, yet scarcely two. If plan- 

 arians are starved or kept in stale water they will divide too 

 often, not getting their growth between divisions, thus pro- 

 ducing planarians which remain pigmies as long as the bad 

 conditions exist. This kind of reproduction can be easily 

 imitated artificially by cutting planarians with a knife (Fig. 

 95). Ways to do this are suggested in the paragraph on 

 aquarium study. 



Habitat. — Turbellarians occur in all kinds of fresh water at 

 any time of year, and their presence can be counted upon in 

 almost any aquatic society. Planarians live in shallow pools, 

 in streams, sometimes very swift ones, and in swampy places, 

 often among sphagnum mosses. A pool which at first glance 

 does not seem to have a planarian in it may shelter a popu- 

 lation of them hiding on whatever they can find there, stones 

 and trash and water plants. Drooping grasses which trail 

 into the water from the bankside may be black with planarians 

 on their under sides. One of the minute turbellarians, 

 Stenostomum (Fig. 96), lives in mats of Spirogyra and other 

 green algse. 



Planarians prefer cool, even very cold water, and temper- 



121 



