106 INVERTEBRATE ZOOLOGY 



Interrelationships of the Classes. — The Turbellaria (Fig. 60) 

 represent the most generahzed group of the Plathelminthes. 

 Modifications in the classes Trematoda and Cestoda represent, 

 chiefly, adaptation to the parasitic habit. Extreme development 

 of organs of generation and fixation (Fig. 63 A) and reduction of 

 locomotor organs, sensory apparatus, and other structures charac- 

 teristic of free existence characterize these two parasitic groups. 



The nemertines are very commonly considered as the highest 

 class of the flatworms. It is coming to be very generally recog- 

 nized, however, that they are more closely allied to the higher 

 worm groups. 



Development. — Some of the flatworms, especially the fresh- 

 water Turbellaria and the monogenetic termatodes, undergo 

 direct development from the fertilized egg. More commonly^ 

 metamorphosis is involved. There are numerous different types 

 of larvae characteristic of the various flatworm groups. Miiller's 

 larva with eight lobes, the margins of which are outlined by a 

 continuous band of cilia, occurs in many polyclads. This seems 

 to represent a primitive type of larva from which many others 

 have been differentiated. 



The pilidium of nemertines characteristically undergoes a 

 complicated metamorphosis during which but a portion of the 

 larva is utilized in the transformation to the adult form while the 

 remainder is cast off. 



Development in the trematodes frequently involves an alter- 

 nation of generations. The generation which lives in snails, 

 and is usually considered as larval (Fig. 65 A-C), undergoes a 

 parthenogenetic cycle of reproduction, while the adults of the 

 same species reproduce the first larval stage (Fig. 65 A) by means 

 of fertilized eggs. 



In addition to the sexual forms of reproduction leading to 

 formation of larvae mentioned above, many of the flatworms 

 reproduce asexually. Some species of Turbellaria undergo 

 transverse fission (Fig. 62). In some of the fresh-water planar- 

 ians (Fig. 61) this seems to be the chief if not the only means of 

 reproduction. This method of naturally reproducing an entire 

 individual from a portion of another rests upon the development 

 of power of regeneration. 



Relationships to Other Phyla. — Through the Ctenophora and 

 the Turbellaria, the two phyla Coelenterata and Plathelminthes 

 seem to have a fairly close phylogenetic relationship. Muscula- 



