APPENDIX TO CHAPTER XV 229 



month by the process of division. Apparently the divi- 

 sion cannot go on indefinitely, but from time to time the 

 Paramecia unite temporarily in pairs and undergo an 

 exchange of some of their nuclear matter. This is doubt- 

 less the beginning of what is known in the higher animals 

 as sexual reproduction. 



APPENDIX TO CHAPTER XV 



KEY TO THE FOUR CLASSES OF PROTOZOA 



a\. Without non-retractile appendages. 



61. With retractile pseudopodia . . -. . . Ehizopoda 



(Ex. Amoeba) 



Z> 2 . Without pseudopodia ; covered with an im- 



perf orate cuticula ; parasites . : . , Sporozoa 



2 . With non- retractile appendages. 



61. No cilia, but with one or more flagella . , Flagellata 



(Ex. Euglena) 



&2' With cilia, or sucking tentacles . ,. . Infusoria 



(Ex. Paramecium) 



KEY TO THE SUBCLASSES AND ORDERS OF INFUSORIA 



a\. With vibratile cilia and no sucking tentacles * - CILIATA 



61. Body everywhere closely beset with cilia. 



GI. No adoral zone . . ... ___ Holotriclia 



(Ex. Parameciurn) 



c 2 . Adoral zone present . ... Heterotricha 



(Ex. Stentor) 



62- Body only partly ciliated. 



d. Cilia limited to ventral side . . . . Hypotricha 



c 2 . Cilia form a circlet around or at upper 



edge of animal . . . . Peritricha 



(Ex. Vorticella) 



a 2 . With sucking tentacles . . . . . . SUCTORIA 



