CHAPTER XXII 



INFLUENCE OF EXTERNAL CONDITIONS ON THE LIFE-CYCLE 

 OF THE ROTIFER; HYDATINA SENTA 



IN the rotifer Hydatina senta, both parthenogenetic and sexual 

 reproduction takes place, and appears to be directly determined 

 in part by external conditions; but also in part by an internal 

 factor, viz. by the early fertilization of certain females. Maupas 

 and later Nussbaum have carried out experiments with this 

 form, and while they agree on certain essential points, especially 

 in regard to the mode of reproduction depending on an external 

 factor, yet Maupas believes the effect is produced by the tem- 

 perature, while Nussbaum thinks that the cause of the result 

 is the food supply, which may be affected by the temperature 

 in the sense that at a higher temperature more food is required 

 than at a lower, because the life processes go on faster. 



The males of Hydatina senta are smaller than the females, 

 measuring .25 millimeter and the females .75 millimeter. The 

 males take no food, and have, in fact, no digestive tract. They 

 are not sexually mature when they, leave the egg capsule, but the 

 sperm ripens during the first day. They live two or three days 

 only, but may fertilize several females during that time. The fe- 

 males require a great deal of food and will die in two days if 

 starved. In five minutes a hungry female will fill her digestive 

 tract, and begin to empty it in 20 minutes. Microscopic organ- 

 isms are eaten, Euglena being the chief food. 



The females may produce the soft-shelled summer eggs or 

 the hard-shelled winter (or resting) eggs. The summer eggs 

 develop without fertilization, and may produce either males or 

 females, one sex or the other being produced by an individual, 

 not both by the same individual. The winter eggs are fertilized ; 



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