FREE-LIVING FLATWORMS 



lack of oxygen. If they creep up and form a line along 

 the water's edge it means that their oxygen supply is running 

 low, more plants must be put in the aquarium or fresh water 

 added every day or two. Once or twice a week they should 

 have a meal of crushed snail-meat, or earthworm, or beef- 

 liver, and the w^ater should be changed the day after feeding. 

 Planarians will live for a long time, three months or more, 

 without any food, but they will become smaller and smaller, 

 using up their own bodies nearly to the vanishing point. 



To test their power of growing new heads and tails it is 

 only necessary to have healthy planarians, a few small, clean 

 dishes so that each animal can have one to itself, and a knife 

 or a pair of scissors. Place a planarian in a shallow dish of 

 water, and press a sharp knife down across it while it is well 

 extended, or lift it out upon wet paper where it can be cut, 

 paper and all, with scissors. Planarians have no blood or 

 blood vessels to be injured by cutting and branches of the 



Fig. 95. — Regeneration in planarians: i, a plan- 

 arian cut crosswise as indicated; 2, the regeneration 

 of the tail ; and 3, the head ; 4, a planarian cut length- 

 wise; 5, the regeneration of the left, and 6, of the 

 right side. 



123 



A 



