I4D ^N INTRODUCTION TO ZOOLOGY. 



this case are called "spores." As for the fusion of 

 two cells, it is said to take place sometimes without 

 the occurrence of subsequent division, the object 

 being merely to produce a stronger cell : this after- 

 wards parts into two, which may sometimes be equi- 

 valent to the original couple : this process is called 

 rejuvenescence. 



With regard to the proper classification of the 

 Protozoa, considerable difiFerence of opinion has ex- 

 isted. Some would prefer to place first, as the most 

 simple aud elementary forms, those kinds which have 

 no nucleus {M.onera of Haeckel, included under the 

 heading of Flacjellata) ; but it is probable that these 

 will eventually be found to be only a stage of some 

 form which, in another stage, does possess a nucleus ; 

 and even if they should be found to be permanently 

 non-nucleated kinds, yet it would still remain to be 

 inquired whether they might not be degenerate types 

 which had lost a nucleus which their ancestors pos- 

 sessed (just as the red blood corpuscles of the mam- 

 malia are degenerate cells which have lost the nucleus 

 possessed by the less highly specialized red blood 

 corpuscles of the other vertebrates), and whether they 

 should not therefore be respectively placed, in classi- 

 fication, near to those forms which they most resemble 

 in general appearance.^ 



' In some classifications, also, there are included with the 

 Protozoa some organisms which are usually supposed to be 

 vegetahJe, e.g., the Volvocinew, often classed with the vege- 



