AGAMIC REPRODUCTION AND REJUVENESCENCE 131 



far advanced and the rate of metabolism is very low. Posterior 

 zooids are not distinctly marked off djTiamically, as in P. doroto- 

 cephala, but the portions which separate are merely small bits of 

 the body at the posterior end which, as the animal becomes pro- 

 gressively weaker, finally cease to be controlled and co-ordinated 

 with other parts by the dominant head-region, and so, sooner or 

 later, react independently and are torn off. In some cases the 

 animal may leave a trail of such fragments behind it as it creeps 

 slowly along. The stimulation resulting from the rupture of the 

 tissues leads to the secretion of shme on the surface of the separated 

 pieces, and this slime hardens and forms a cyst within which the 

 pieces gradually undergo reconstitution to whole animals of small 

 size which sooner or later emerge. 



Fragmentation may continue until only the head and a short 

 piece of the body two or three millimeters in length remain, or it 

 may be confined to the posterior third or half of the body. After 

 fragmentation is completed, the anterior piece, whether large or 

 small, may encyst, or it may remain more or less active and grad- 

 ually undergo reduction in size in consequence of starvation. 

 Finally, after considerable reduction has occurred, it develops a 

 new pharynx and mouth and a new posterior end, and begins to 

 feed and grow again. Cases of this sort will be considered in 

 chap. vii. 



The encysted fragments do not withstand complete desiccation, 

 but the bottoms of the ditches and pools in which they live retain 

 sufl&cient moisture to keep them alive. In the autumn the ditches 

 do not usually fill again before cold weather, although they may do 

 so, in which case the worms may emerge from the cysts at that 

 time, but their growth is soon stopped by low temperature. Com- 

 monly, however, they appear only in spring, as soon as the ditches 

 thaw out. This cycle is repeated year after year, and thus far 

 neither sexually mature animals nor animals with any part of the 

 sexual ducts or copulatory organs have ever been found, though 

 ovaries and testes in early stages of development may sometimes 

 be present. 



In the laboratory the animals may pass through the whole life 

 cycle in two or three months, for the encysted fragments when 



