328 FRESH-WATER BIOLOGY 



and Planaria agilis sexual organs begin to develop early in the 

 autumn and mature in the spring. After the cocoons are depos- 

 ited the reproductive organs degenerate and reproduction is again 

 carried on by transverse division into two pieces with subsequent 

 regeneration of the missing parts in each piece. The division 

 plane in most planarians passes just back of the pharynx. In 

 Planaria velata there is a division into pieces of various sizes 

 which encyst in a slime layer in response to unfavorable con- 

 ditions. This slime layer hardens into a shell-like covering. 

 Entire animals may also encyst. Asexual reproduction among 

 planarians may occur at any time of the year and in many species 

 is the usual method of propagation. The factors which control 

 the development of sexual maturity are not fully understood 

 although the food supply unquestionably plays an important part. 



Turbellaria undergo no metamorphosis during development but 

 emerge from the egg, resembling the parent except in the lack 

 of sexual organs. In viviparous forms the young develop within 

 the mesenchyma of the parent and make their way to the exterior 

 through the body wall in the posterior region. 



Flatworms are extremely responsive to external influences and 

 the larger forms especially give interesting and specific reactions 

 to various kinds of stimuli. If a dish in which they are quietly 

 gliding about is jarred even very slightly, it will cause them to 

 stop and contract until quiet is restored, or if at rest and the dish 

 is moved they respond by becoming active as soon as the disturb- 

 ance ceases. Violent disturbance induces a highly excited condi- 

 tion with a loss of their more delicate reactions. After being 

 disturbed the animals continue moving about for some time, this 

 period depending on the strength of the stimulus and the physi- 

 ological condition of the animal. Naturally it depends also upon 

 the species since some are more active than others. They come 

 to rest in some sheltered spot, normally in groups. Light plays 

 an important part in determining their resting place as they show 

 decided negative photokinesis. The length of time of the resting 

 period varies greatly. The animals are much more active at 

 night than in day time; this is probably due to their feeding 

 habits. 



