PROTOZOA TO WORMS 41 



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coloring matter of plants. It lives therefore on un- 

 organized fluid nourishment, carbon dioxide, nitrates, 

 etc. It is a plant. But certain characteristics render 

 it probable that it once lived on solid food and was 

 therefore an animal. For where almost the sole 

 difference between plants and animals is in the fluid 

 or solid character of their food, a change from the one 

 form into the other is not as difficult or improbable as 

 one might naturally think. And plants and animals 

 are here so near together, and travelling by roads so 

 nearly parallel, that, even if volvox never was an 

 animal, it might still serve very well to illustrate a 

 stage through which animals must have passed. 



The cells of volvox do not form a solid mass, but 

 have arranged themselves in a single layer on the out- 

 er surface of the sphere. For a time, under favora- 

 ble circumstances, volvox reproduces very much like 

 magosphsera, and each cell can give rise to a new, 

 many-celled individual. But after a time, especially 

 under unfavorable circumstances, a new mode of re- 

 production appears. Certain cells withdraw from the 

 outer layer into the interior of the colony. Here they 

 are nourished by the other cells and develop into true 

 reproductive elements, eggs and spermatozoa. Fertili- 

 zation, that is, the union of egg and spermatozoon, or 

 mainly of their nuclei, takes place ; and the fertilized 

 egg develops into a new organism. But the other cells, 

 which have been all the time nourishing these, seem 

 now to lack nutriment, strength, or vitality to give rise 

 to a new colony. They die. 



We find thus in volvox division of labor and cor- 

 responding difference of structure or differentiation ; 

 certain cells retain the power of fusing with other cor- 



