THE LIFE HISTORY OFPLANARIA VELATA

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Abstract
This form ordinarily undergoes an asexual growth followed by fragmentation and encystment of the separate fragments. High temp. accelerate fragmentation and encystment or promote their occurrence when the animal is of a smaller size. Low temp. prevent encystment or inhibit it until the worms are larger than the normal. In nature, under conditions of decreasing temp. and abundant food, sex organs are developed; these disappear under laboratory conditions, their disappearance being hastened by high temp. and retarded by low. Under constant temp. sex organs are never developed in laboratory stocks and the life-history is entirely asexual. Experimentally asexual stocks become sexual if there is a decrease in temp. during the growth period.

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