358 ABSTRACTS OF PAPERS. [\'OL. II. 



(Zacharias, Zeit.f. Wiss.Zool., 1886, Bd. XLIII, pp. 271-275), 

 where there is some rearrangement of the gut and the pharynx 

 is partly developed before separation ; (4) Planaria fissipara 

 (Kennel, Zool.Jahrb., Abth. f. Anat. u. Ontog., 1888, Bd. Ill, 

 pp. 447-486), in which a complete worm is developed out of 

 the posterior third of a large specimen and both reach normal 

 proportions before separation. In the last three cases the 

 division occurs at a corresponding place. 



The division in Planaria macnlata seems to be brought 

 about by a contraction of the circular muscles, which pinches 

 the individual in two a short distance behind the pharynx. 

 The cut ends of either piece are as though they had been 

 produced by a knife-cut, and examination of sections shows 

 that the parenchyma at the scar is actually naked. There is 

 nothing like a furrow on the outside previous to the division, 

 which, nevertheless, is a regular and normal reproductive 

 process and not induced by any ordinary irritation or mutila- 

 tion of the animals. The large number of pieces in various 

 stages of regeneration, found in collecting, is sufficient evidence 

 of the occurrence of the fission under natural conditions. 



Worms will not divide in the laboratory to any considerable 

 extent unless well fed. If the water has become foul and is 

 replaced by fresh, a considerable number of specimens will 

 usually be found divided within the next twelve hours. The 

 division usually occurs at night, whether the dishes are shaded 

 or not. The morpholaxis of the head and tail pieces resulting 

 from a division is rapid. Tail pieces may reach almost the 

 normal proportions and re-divide in from five to six days if 

 well fed, heads in not less than ten days. There is no regular 

 interval between the divisions. 



In certain localities this species does not possess reproduc- 

 tive organs at any time during the year, but has during the 

 summer months an active period of asexual multiplication. 

 In other localities the worms develop these organs in the fall 

 and lay eggs in the spring ; and although all the specimens are 

 without these organs at the end of the summer, asexual repro- 

 duction has never been observed. In another sexual locality 

 the worms, when they are without reproductive organs at the 



