Japanese Freshwater Triclads. CY 



PI. <jonocej)hala, as observed by me. seems also to reproduce to 

 a. large extent by fission. I have never yet met with individuals 

 which were doubtlessly quite in the process of dividing, except 

 showing a dark pigment spot in the middle of the dorsal surface, 

 but those bearing evident marks of foregone fission are rather 

 common. To judge from these, it seems that this always takes 

 place at a point immediately behind tlie pljaiynx or a short dis- 

 tance farther back, much as is known from PL macnlaia Curtis. 

 Instances of division in front of the pharynx, known to have taken 

 place in PI. alblsima Sp:kera, was never observed in the species in 

 question. The severed end of the tw^o pieces produced by division 

 presents for some time a transverse and nearly straight edge, expos- 

 ing the parenchyma in a thin white line. For some time after 

 the healing of the wound there is observed at the end of the body 

 concerned a colourless area of regeneration, which area, bounded 

 off from the old parts by a zone of concentrated pigmentation, is 

 at first of a crescentic, and then more or less of a tiiangular shape. 

 In the tail piece that area develops into the part uf tlie new head 

 anterior to the auricles, wdiile in the head piece it merely forms 

 the new^ tail-end. In the former case the regenerated area increases 

 considerably in extent, and the eyes appear as two minute dots 

 close to its border against the pigmented old parts of the ])ody. 

 The points of the auricles begin to show themselves somewhat 

 later. 



In all the species mentiolied, reproduction by fission takes 

 place most actively during June, July and August, in wdiich period 

 it is exceedingly rare, if at all, to discover individuals with de- 

 veloped sexual organs. So far as my observations go, the fission 

 ceases in the autumn and then the reproductive organs begin to 

 develop. It may tlierefore be said, in unison witli Cuktis (2U), 

 that the life history of these planarians presents alternate periods 

 of asexual and sexual reproduction. It is rather difticult to under- 

 stand why the lowering of tlie temperature in the fall induces an 

 acceleration in the growth of the sexual elements. There are 

 perhaps two theories to be offered in explanation. One is that the 

 planarians which in their primitive habitation lived in cold regions 



