ASEXUAL CYCLE OF PLANARIA VELATA. 195 



soon hardens into a tough membrane and forms the cyst. It is 

 a familiar fact that stimulation is often followed in the turbellaria 

 by the secretion of a large amount of slime. That is exactly what 

 occurs in these pieces and in this species the slime hardens and 

 forms the cyst. Apparently then the encystment of pieces in 

 Planaria velata is simply the result of a sudden stimulation. Any 

 factor that increases the stimulation increases the frequency of 

 encystment. 



As regards the greater frequency of encystment with advancing 

 age of the worms, I have not been able to reach a definite con- 

 clusion based on experiment, but my observations indicate that 

 old worms secrete more slime on stimulation than young. Ap- 

 parently the gland cells either increase in number or the quantity 

 of the substance in them which produces the slime increases as 

 the animals grow older. 



When the slime which produces the cyst first appears it is soft 

 and an active whole animal is able to creep out of it without 

 difficulty, but the pieces are much less active and do not succeed 

 in escaping from it before it hardens. If the cysts are carefully 

 opened with needles soon after they are formed and the pieces 

 removed without injury or any great degree of stimulation they 

 usually do not encyst again but develop into whole worms 

 while free. But if they are injured or otherwise strongly stimu- 

 lated they commonly encyst a second time. 



In short all the facts indicate that encystment of pieces is 

 merely a result of the stimulation accompanying section. It is 

 not an adaptation to conditions or a preparation for the future 

 in any sense. The animals do not encyst because they usually 

 live in temporary bodies of water but they are able to live under 

 these conditions because they encyst. 



V. THE PROCESS OF FRAGMENTATION IN OLD WORMS. 



The process of fragmentation in nature is very evidently 

 similar in character to the process of zooid-formation and fission 

 in Planaria dorotocephala and P. maculata (Child, 'lie}. In 

 consequence of increase in length of the body and the decrease 

 in rate of metabolism as the animal becomes older the posterior 

 regions of the body usually become to some extent physio- 



