AGE DIFFERENCES IN METABOLISM OF PLANARIA. 



397 



and smallest worms was about 40 per cent. (Table I., decapitated 

 worms). 



III. EXPERIMENTS WITH Planaria velata. 

 Planaria velala lives in the Chicago region in temporary ponds, 

 usually those that are passing into prairie, known to ecologists as 

 "prairie ponds." Like the preceding species it is never found 

 sexually mature, and reproduces exclusively by a peculiar asexual 

 method. After the worms have attained a certain size, pieces 

 drop off from the posterior end, surround themselves with 

 mucus, and pass into an encysted condition. This process con- 

 tinues until the entire worm has formed a series of cysts. Within 

 the cysts, the pieces undergo regeneration into complete worms 

 of very small size, and these emerge from the cysts in about four 

 weeks. This cycle is repeated as long as the ponds contain 

 water; after the water dries up, the cysts remain quiescent until 

 the following spring. 



TABLE II. 



COMPARISON OF THE RATE OF OXYGEN CONSUMPTION OF LARGE AND SMALL INDI- 

 VIDUALS OF Planaria velata. 



Three Lots of Worms 10-12 mm. Long, before Rncystment; Collected March 7, Last 

 Fed March 10, Tested 13. Temp. 22 C. 



Three Lots of Worms less than 4 mm. Long, Emerged during May from Cysts Formed 



by Above Lots of Worms; Fed Several Times after Emergence; Last 



Feeding June 6, Tested June Q. Temp. 22 C. 



Owing to the nature of the life cycle of this animal, it was not 

 possible to test the large and small worms simultaneously but the 

 large worms were tested before encystment, the small ones after 



