THE FLATWORMS 



185 



sensitive nature. Reproduction is by fission, as in Nais (p. 176), 

 or by the sexual method, and each individual possesses both 

 male and female organs, i.e. is hermaphroditic. The reproduc- 

 tive organs may be located easilyTn figure 105T 



Regeneration. — Planarians show remarkable powers of 

 regeneration. If an individual is cut in two (Fig. 106, A), the 

 anterior end will 

 generate a new tail 

 (B, B 1 ), while the 

 posterior part de- 

 velops a new head 

 (C, C 1 ). A cross- 

 piece (D) will gen- 

 erate both a head 

 at the anterior end, 

 and a new tail at 

 the posterior end 

 (D-D 4 ). The head 

 alone of a plana- 

 rian will grow into 

 an entire animal 

 (E-E 3 ). Pieces cut 

 from various parts 

 of the body will 

 also regenerate 

 completely. No 

 difficulty is experienced in grafting pieces from one animal upon an- 

 other, and many curious monsters have been produced in this way. 



The power to renew lost parts by regeneration is of great 

 importance to these animals, since their soft bodies are often 

 injured by the rocks in the streams among which they five. 



Parasitic Flatworms. — Besides Planaria and other free- 

 living flatworms that belong to the class Turbellaria, there are 

 two classes of parasitic flatworms, the Tr'ematoda or flukes and 

 the Cestoda or tapeworms. 



Regeneration of planaria. 



A, normal worm ; B, B 1 , regeneration of anterior 

 half ; C, C 1 , regeneration of posterior half ; D, cross- 

 piece of worm ; D 1 , D 2 , D 3 , D 4 , regeneration of same ; 

 E, old head ; E 1 , E 2 , E 3 , regeneration of same ; F, F>, 

 regeneration of new head on posterior end of old head. 

 (From Morgan.) 



