The head of the fresh-water planarian, Dugesia ti- 

 grina, shows two pigment-cup eyes and a pair of sen- 

 sory lobes. (P. S. Tice) 



everywhere, and may also be found in cold mountain 

 streams. In the warm waters of the tropics or sub- 

 tropics they appear to be scarce. Though the usual 

 size range of these worms is from '/s of an inch to 1 

 inch or so long, there are 4-inch planarian giants in 

 Lake Baikal in East Siberia. Common colors are 

 white, gray, brown, or black, sometimes spotted, 

 streaked, or striped. In the most familiar genus, Dii- 

 gesia, the head is triangular with two prominent little 

 sensory lobes that detect chemicals, food, touch, and 

 water currents. Most fresh-water planarians, how- 

 ever, have blunt heads which lack conspicuous lobes, 

 though the sides of the head serve the same sensory 

 functions. Dugesia and others have two eyes, each 

 consisting of a pigment cup that shields light-sensitive 

 cells in all directions but one, enabling the animal to 

 respond to the direction of the light that strikes the 



eye. A few planarians have two clusters of tiny eyes; 

 and many are wanting in specialized eyes, though 

 the bodies may be generally sensitive to light. 



Planarians can be amusing and undemanding ani- 

 mals to keep and to watch. Dugesia (formerly Eu- 

 pkinaria), which has many species in Europe, Asia, 

 and the Americas, can be collected in ponds or 

 springs. Almost any spring or spring-fed marsh that 

 supports watercress will have a lively population of 

 worms moving about the vegetation or clinging to the 

 underside of almost any stone one may overturn. A 

 piece of raw meat, beef liver, or fish, strategically 

 placed in such a spring, will bring worms from their 

 hiding places by the hundreds, and they can be seen 

 gliding smoothly upstream to the bait, guided by the 

 meat juices in the current. A few worms can be easily 

 maintained in a small bowl filled with bottled spring 

 water or clear pond water. Tap water in most places 

 is too toxic; and the worms will not flourish in an 

 overheated room. They move about on the bottom 

 and sides of the bowl in a slow glide, with the head 

 bending from side to side as though testing what lies 

 ahead. They will be seen to move toward the dark 

 side of the bowl and to remain whenever possible in 

 contact with some solid surface or film. Planarians 

 do not swim freely through the water, but if they 

 have been moving along the underside of the surface 

 film they will leave the surface by gliding down at- 

 tached to a strand of mucus. When the worms are not 

 being observed the dish should be kept loosely cov- 

 ered to reduce evaporation and to protect the worms 

 from strong light. 



Bits of beef liver are most convenient for feeding 

 many kinds of planarians; the food may be left with 

 the worms for several hours, but then it must be re- 

 moved, the bowl rinsed, and the water changed. 

 When actively feeding, Dugesia extends its agile, tu- 

 bular pharynx through the mouth opening, near the 

 middle of the under surface. Sucking movements of 

 this feeding tube break up the food into microscopic 

 bits which can be swallowed along with the oozing 

 juices. If unfed for weeks or even months the worms 

 will not die but will use up their reserves and become 

 smaller and smaller. 



Dugesia, and several other common genera as 

 well, are noted for multiplying by asexual rupture of 

 the body, usually just behind the region which houses 

 the feeding tube. Dugesia tigrina, the commonest 

 species of the United States, breeds sexually only in 

 early spring and summer, fastening its stalked egg 

 capsules under stones. The rest of the year the worms 

 reproduce only by asexual rupture; and in some 

 places Dugesia seems to be permanently asexual. 

 Those planarians that multiply by natural rupture 

 have extraordinary powers of regeneration. Almost 

 any piece of moderate size cut from a Dugesia will 

 reorganize itself into a complete worm. If the worm 



122] 



