259 



a 



organs; this necessarily leaves the exact relationship of this species 

 somewhat doubtful. 



These planaria are extremely delicate and more difficult to keep in 

 aquaria than any other species with which I am familiar. They secrete 

 an exceptionally large amount of slime which causes the water to be- 

 come impure quickly. Exposure to the air for a short time causes the 

 formation of a heavy slime cyst similar to that described by Child (1901) 

 for the nemertean Stichostemina. Irritation often causes a similar result. 

 When disturbed these planaria continue in motion about as long as 

 P. agilis but make much less use of "testing" re- 

 actions ^ and change their course much less fre- 

 quently. 



A remarkable characteristic of P. velata is what 

 I shall call fragmentation. It appears to be the nor- 

 mal method of reproducing asexually. Worms kept 

 in aquaria were found to have divided transversely 

 into a number of small pieces. Isolated specimens 

 divided into a variable number of pieces depending 

 somewhat on the length of the specimen. The lar- 

 gest number of pieces obtained from one planarian 

 was thirteen. The average number was from five to 

 seven. In all cases observed, fission began at the 

 posterior end and proceeded towards the anterior. 

 Single pieces were pinched of£ quickly and in a way 

 similar to that in which fission among planaria or- 

 dinarily occurs. The time for the entire fragmen- 

 tation of one individual varied from less than an 

 hour to four days. When the process was prolonged 

 the pieces usually lacked vitality and soon died. 

 Fragmentation usually took place at night and in- 

 volved the entire worm, though in some cases from 

 one to three pieces only separated from the posterior region, leaving the 

 larger part of the worm undivided. The pieces, with the exception of 

 those from the ends were very uniform in appearance. Immediately after 

 pinching of£ they became quite rounded and showed peculiar rhythmical 

 contractions which continued until regeneration was well started. It was 

 very difficult to keep the pieces alive because of the extreme delicacy 

 of the species. Slime cysts were often secreted by the pieces when 

 disturbed. Many pieces from the aquaria showed various stages in re- 

 generation, but of those obtained from isolated specimens, only one 

 remained alive until fully regenerated. 



Fig. 1. Planaria ve- 

 lata n. sp. From life. 



17=) 



