248 



ELEMENTS OF BIOLOGY 



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into a small planarian. The walls of the capsule resist drying; so 

 in Nature the developing young are protected and life is carried 



over until conditions are suitable for the break- 

 ing of the capsule and the release of the young 

 animal. 



Planaria dorotocephala is another fresh water 

 flatworm found in spring-fed pools in the Middle 

 West. So far as is known it never reproduces 

 sexually in Nature. In fact, animals with mature 

 sex cells have never been collected. Its method 

 of reproduction together with its characteristics 

 of growth and starvation comprise a most in- 

 teresting adaptation to its environment. It is 

 known to feed on the blood and juices of injured 

 or freshly killed animals. In the late Summer 

 the food supply is abundant and the planarians 

 grow to their maximum natural size. They 

 become sluggish and eventually pull into two 

 parts (Fig. 170), the posterior fragment devel- 

 oping a new head and the anterior fragment 

 developing a new posterior end. The two prod- 

 ucts of this fission soon become identical and 

 by the growth resulting from feeding again 

 assume a size that brings about another fission. 

 Thus the number of individuals is increased dur- 

 ing the Summer. But in Autumn the food supply 

 decreases and largely disappears during the Win- 

 ter. During this season the animals subsist largely 

 upon their own body contents, decreasing in size 

 with prolonged starvation. Thus in the Spring 

 one ordinarily collects only small animals and 

 during this season fission occurs infrequently in Nature. With the 

 appearance of adequate food supply these starved forms, which are 



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Fig. 170. — A, a 

 planarian worm 

 about to undergo 

 fission; the posterior 

 region is potentially 

 a new individual, 

 B, another type of 

 free living flat- 

 worm, Stenostotn- 

 um, showing new 

 individuals already 

 distinctly formed 

 before separation 

 from the parent. 

 Compare with the 

 agamic reproduc- 

 tion in plants shown 

 in Fig. 172 and 

 with agamic repro- 

 duction of yeast 

 plants, Fig. 24. (B 

 ^fter Child.) 



