502 EVOLUTION OF ANIMALS Part V 



the underlying surface and the body is moved forward in rapidly repeated 

 microscopic lurches that merge into a glide. 



Feeding. The majority of turbellarians are carnivorous. The smaller fresh- 

 water ones feed upon crustaceans and worms that are nearly microscopic, the 

 larger ones on snails, earthworms and insect larvae, often on their softened 

 remains. Even in quiet waters they can detect juicy meat two or three feet 

 away. As the worm recognizes the food it pauses, swings its raised head about 

 and starts directly toward it. First it touches, then rubs its head against the 

 piece and glides onto it, finally stretches and dips its pharynx into it (Fig. 25.2) . 



Digestion, Assimilation and Food Storage. Flatworms are strikingly different 

 from other bilaterally symmetrical animals in having the mouth half way 

 down the body, curiously enough not in the important head region (Figs. 

 25.3, 25.4, 25.5). The pharynx leads into the three-forked (in triclads) 

 intestine whose many branches reach throughout the body. Practically any 

 piece that may be torn from the body takes digestive and excretory cells with 

 it; thus it can be nourished and can grow. 



Feeding experiments and microscopic examinations of the intestine have 

 shown that the entire processes of digestion, absorption, assimilation and 

 storage of food occur within the partly ameboid cells of the intestinal lining. 

 A planarian grows fat in its linings, usually of the intestine; food stored there 

 is largely fat, rarely glycogen. Nothing is known of the actual processes by 

 which the stored food is transferred and used by the other cells of the body. 

 In one series of experiments, planarians (Dugesia) were starved for two 

 weeks, then fed on beef liver. At frequent intervals, some of them were killed 

 and examined microscopically. The partially ameboid cells began to engulf the 

 bits of liver as soon as they came in contact with them. Swollen with absorbed 

 fluid, they bulged into the intestine and embraced the food with their pseu- 

 dopodia. Within them the bits of food and fluid were digested in food vacuoles 

 like those of amebas. It took about eight hours for the content of a full in- 

 testine to be taken up by the ameboid cells. During digestion planarians take 

 in two or three times more oxygen than usual and utilize the stored fat for the 

 extra energy expended. 



Fresh-water planarians can endure starving for six to 14 months but at the 

 end of that time they may be reduced to one three-hundredth of their original 

 size. The greatest degeneration is in the reproductive system, part of which 

 entirely disappears. Their condition suggests that of worker honeybees that 

 are chronically underfed and have undersized reproductive organs. The heads 

 of starved planarians are relatively large because the nervous system is not 

 reduced. 



Respiration. Oxygen and carbon dioxide are exchanged by diflfusion through 

 the body as in ordinary aerobic respiration, a contrast to the anaerobic respira- 

 tion of parasitic flatworms (see cestodes p. 515). 



